The phrase Mother of the House, also Mother of the House of Commons, designates the longest serving female member of the British House of Commons.

This phrase occurs, for example, in Keir Starmer’s cabinet will have most female ministers in history, by Aletha Adu and Michael Goodier, published in The Guardian (London and Manchester, England) of Friday 5^th July 2024:

The Labour veteran and Britain’s first black female MP, Diane Abbott ¹, will become mother of the house in the new parliament, having served her Hackney North and Stoke Newington constituency for almost 40 years.

¹ The Labour politician Diane Abbott (born 1953) has served as Member of Parliament for Hackney North and Stoke Newington since being elected in 1987.

The phrase Mother of the House (of Commons) was coined after Father of the House (of Commons), which designates the longest serving male member of the British House of Commons.—This is implied in the following early, isolated (and rather odd) occurrence of Mother of the House of Commons, from one of the unconnected paragraphs making up the column Charivaria, published in Punch, or the London Charivari (London, England) of Wednesday 29^th May 1907 [Vol. 132, page 379, column 1]:

In consequence of an oversight the rumour has got about that Sir Henry Campbell-Bannerman ² has become the Mother of the House of Commons.

² Henry Campbell-Bannerman (1836-1908), Prime Minister of the United Kingdom from 1905 to 1908, became Father of the House of Commons in May 1907.

The phrase Mother of the House (of Commons) was originally applied to the U.S.-born British politician Nancy Astor (née Nancy Witcher Langhorne – 1879-1964), Viscountess Astor, who—following the Parliament (Qualification of Women) Act 1918—was the first woman to sit in the British House of Commons, serving as Member of Parliament for Plymouth Sutton from 1919 to 1945.

The earliest occurrences that I have found of the phrase Mother of the House (of Commons) are as follows, in chronological order:

1-: From The Leeds Mercury (Leeds, Yorkshire, England) of Saturday 24^th January 1920 [No. 25,047, page 5, column 3]:

“POOR THING!”
Mrs. Fisher ³ on Lady Astor in the House.

Mrs. H. A. L. Fisher, speaking in Leeds last night at a meeting of the Yorkshire Council of Societies for Equal Citizenship, made a stirring appeal for more women Members of Parliament.
Women, she said, were not in the least content with the vote.
It was very nice during the war to be told how useful women were, but now they were beginning to be regarded as a nuisance again. They were, however, quite prepared to be “splendid” again; and she supposed that it was inevitable that men should dislike women for doing the things they had done.
So far, she said, there was one lady member on the floor of the House, and it was a thing to be thankful for. “Since the poor thing did get in,” Mrs. Fisher added, “you should see her post-bag.” It was completely impossible for Lady Astor to do the whole of woman’s work in the House. She was called “The Mother of the House of Commons.”
Her correspondence was so tremendous that she had to engage fresh secretaries every minute, and she had queues of them coming with letters they couldn’t possibly deal with.
Lady Astor had pushed open the door, and she hoped the women of the North would send their representatives to Westminster.

³ Lettice Fisher (née Lettice Ilbert – 1875-1956) was a British educator, economist, suffragist, and founder of the National Council for the Unmarried Mother and her Child.

2-: From a debate on the Temperance (Wales) Bill that took place in the House of Commons on Friday 26^th March 1920—from Hansard, the official report of all Parliamentary debates:

Mr. Pemberton Billing ⁴:
The hon. Member for Plymouth (Viscountess Astor), who spoke in support of this Bill, presumably with full authority and with the support of the Movers and Seconders of it, told us that what they wished to do was to nationalise drink, and, speaking as the mother of the House, she told us exactly what she proposed to do with us all, even if we did not agree. If I may give a little advice, speaking as a relatively young man to the Mother of the House of Commons, it would be to say that men are sometimes, are always, following the wishes of their womenkind, but never when they are previously expressed.

⁴ The British aviator, inventor and publisher Noel Pemberton Billing (1881-1948) served as Member of Parliament for Hertford from 1916 to 1921.

3-: From the Westminster Gazette (London, England) of Friday 8^th June 1923 [No. 9,881, page 6, column 6]:

The “Mother” of the House.

Immediately after questions in the House of Commons to-day, Lady Astor simply raced into the Lobby to greet the new member, and they talked together for some minutes as if the scores of people waiting to shake hands were non-existent.
Lady Astor is of course the “mother” of the House just as Mr. O’Connor ⁵ is “father,” and it fell to her to-day to show Mrs. Philipson ⁶ round—particularly to the room downstairs that is called “the lady members’ room.”
Mrs. Wintringham ⁷ had an engagement of long standing in her constituency to-day, and she thoughtfully wrote a little note to the new hon. member explaining her absence.

⁵ The Irish nationalist politician Thomas Power O’Connor (1848-1929) served as a Member of Parliament for nearly fifty years.
⁶ The British actress Mabel Philipson (née Mabel Russell – 1886-1951) served as Member of Parliament for Berwick-upon-Tweed from 1923 to 1929.
⁷ Margaret Wintringham (née Margaret Longbottom – 1879-1955) served as Member of Parliament for Louth, Lincolnshire, from 1921 to 1924.

4-: From Lady members. Why there are not more, by James Myles Hogge (1873-1928), Member of Parliament for Edinburgh, published in the Birmingham Gazette (Birmingham, Warwickshire, England) of Thursday 14^th June 1923 [No. 24,720, page 4, column 4]:

The third lady member of the House has arrived. It has been remarked already, of course, that no lady member has yet reached Westminster, as it were, on her own. The way to Westminster is apparently only paved with a husband’s achievements. In the case of all three lady members access to the House has been inherited.
Curiously enough, the result has not only been satisfactory but good, for all three ladies concerned are competent, able, and informed women.
Lady Astor, the Mother of the House of Commons, is a piece of quicksilver. Her brain is as active and agile as her movements, and she is never still a single moment. Yet her purpose is high, and she can always be relied upon to make not only a good but an arresting speech. She is over-prone to interrupt, and is impatient with the rather slow and cumbrous methods of the House, but she can very easily be forgiven much.

The phrase Mother of the House (of Commons) was, naturally, applied to Nancy Astor until her retirement in 1945—as illustrated by the following from a debate on the rebuilding of the House of Commons that took place on Thursday 25^th January 1945—from Hansard, the official report of all Parliamentary debates:

Viscountess Astor:
I am glad that the last speaker talked about this as the Mother of Parliaments. I am in a sense the mother of this House, and I wish that the House would take my advice. I hate saying a word against the Prime Minister ⁸ at this stage of the war. […] A great deal is owed to the Prime Minister, particularly by the House of Commons, and I hate to disagree with him in any way. I do, however, disagree with him in wanting the new Chamber kept as it was. There was nothing very comfortable about the old House or even very historic. It was a brand-new House built at the worst period of English architecture. […]
[…] I feel that the Prime Minister, with all his vision, is looking backward instead of forward in regard to the new House of Commons. When I first got into the House, one had to get here at 7.30 in the morning and wait for hours to get a seat, and then often did not get one. The only thing with which Members were completely supplied was drink. That was the only thing one could get at any time. That may be good or not, but it did not appeal to the mother of the House. No mother wants the most comfortable place in her home to be the place where the children can drink at any time of the day or night.

⁸ This refers to Winston Leonard Spencer Churchill (1874-1965), Prime Minister of the United Kingdom from 1940 to 1945.