The British-English phrase (still) going strong, like Johnnie Walker, also like Johnnie Walker, (still) going strong, is an extended form of going strong, meaning continuing to be healthy, vigorous or successful.

This phrase originated in advertisements for Johnnie Walker, a brand of Scotch whisky; these advertisements, which first appeared in 1908, featured:
– the figure of Johnnie Walker as a striding, cheerful Regency man, created by the English cartoonist, painter and illustrator Thomas Arthur Browne (1870-1910);
– the slogan (born 1820) still going strong.

The following advertisement was published in The Tatler (London, England) of Saturday 2^nd December 1908:

Johnnie Walker
For Christmas

The Christmas Gift problem is solved by Johnnie Walker. Johnnie Walker has been the Christmas standby for more than eighty years.
You can pay your friend no higher compliment than to suggest that his palate is educated to the Johnnie Walker standard of Scotch whisky.

EXPERTS DISAGREE.
Even experts disagree as to the age at which whisky is at its best. In Johnnie Walker you have your choice of three guaranteed ages:—White Label, over six years old; Red Label, over ten years old; Black Label, over twelve years old.
More than three-and-a-half million gallons of pure malt Scotch whisky are perpetually in bond—ageing—as a positive safeguard against any immature Johnnie Walker being put on the market. This is the largest reserve stock of pure malt Scotch whisky held by any one firm, and is the bonâ fide property of

JOHN WALKER & SONS, Ltd.,
Scotch Whisky Distillers,
KILMARNOCK.
Born 1820
Still going strong

After Thomas Browne died, several illustrators successively drew the figure of Johnnie Walker. During the First World War (1914-18), it was Leo Cheney (1878-1928), and the advertisements were patriotic. For example, the following was published in The Globe and Traveller (London, England) of Wednesday 16^th December 1914:

A group of people are rejoicing at a poster that reads:

WAR NEWS.
OFFICIAL
BRITISH ARMY
STILL GOING
STRONG

Striding away from the crowd, Johnnie Walker says:

“‘Still going strong’: that’s the slogan. It’s been mine since 1820.”

The earliest occurrence of the phrase that I have found is from this advertisement, published in The Stage (London, England) of Thursday 22^nd September 1910:

WANTED, by Mlle. FLORENCE, the Famous London-to-Brighton Globe Walker, a Few Good Theatres and Music Halls for one of the Finest Variety Companies Touring. I travel Artists only, who know their business and do it. My Printing is all paid for when it arrives at the theatre. My Company do not require bailing out of the station. I have Printing from 18-sheet down to D.C., and will supply as much as you can post. If I cannot draw money into your Theatre or Music Hall it is time you closed up. I have the Greatest Outside Attraction in the business—a Novelty that has earned £500 in one week. This is not a picture show, but a Variety Show, consisting of the Best Artists in the business. Once or twice nightly. This is the fifth year of my Tour in all the Principal Theatres and M.H. in Great Britain, and, like Johnny Walker, still going strong.
Vacant November 14^th, 21^st, 28^th, and January Dates.
All communications to MADAME FLORENCE, Bostock’s Hippodrome, Glasgow.

The second-earliest occurrence of the phrase that I have found is from The Port-Glasgow Express and Observer (Port Glasgow, Renfrewshire, Scotland) of Friday 30^th January 1914:

What the Folks are Saying.
[…]
That two anniversaries fell to be celebrated in the Port on Sunday.
That one was Rabbie Burns and the other ex-Provost Fyfe.
That the latter being only one year short of the three-score-years and ten, is almost double that of the poet at his latter end, and, like Johnnie Walker, still going strong.

The following is from The Ealing Gazette and West Middlesex Observer (Ealing, Middlesex, England) of Saturday 3^rd October 1914:

From her son, Drummer F. Woodley, of the 4^th Battalion Royal Fusiliers, who is serving with the Expeditionary Force, Mrs. Woodley, of 6, Northfield-road, West Ealing, has received the following postcard:—
“Just a line or two to let you and all at home know that I am quite well, and still going strong, like Johnnie Walker! Pleased to say that I have received Reggie’s letter quite safe, dated September 4^th. You need not worrie [sic], mother, as everything is A1, and we are always merry and bright. Give my love to all; in haste—Fred.”

From Thursday 8^th October 1914 onwards, The Magnet Film Co., Ltd., a film-producing company located in London, used the phrase in advertisements for Humanity; or, Only a Jew (1913), by John Lawson (1865-1920)—advertisements published in The Bioscope (London, England), a weekly intended for the professionals of the nascent film industry:

HUMANITY BY JOHN LAWSON
Length 3,000 ft.                     Like “Johnnie Walker”—still going strong.

The following is from the Burton Daily Mail (Burton upon Trent, Staffordshire, England) of Wednesday 10^th November 1915:

Mr. D. Rutherford, Schoolmaster, of Tutbury, has received a letter from Regt. Quartermaster-Sergeant E. Florence, now on the 7^th North Staffords, who is out in Gallipoli. Writing under date of October 20^th he says: “[…] I was in orders as Reg. Quartermaster-Sergeant (August 20^th), which office I am proud to say I still hold. Of course my duties now are to see to the supply of rations and ammunition from the supplies to the firing line. I get plenty of thrilling journeys, but, like the old famous Scotch (“Johnnie Walker”) I am still going strong.”