The jocular phrase Tom Mix in Cement is a retort to the question what’s on at the pictures?.

With a pun on to mix cement, this phrase refers to the U.S. film actor Thomas Edwin Mix (Thomas Hezikiah Mix – 1880-1940), who starred in many early westerns from 1909 to 1935—as explained by Harry Cronin in As Slab-Happy As Ever: For a quarter century, fans have been revering Hollywood stars’ imprints in cement, about “Grauman’s Chinese Theatre in Hollywood, where stars leave their marks in cement slabs”, published in the Daily News (New York City, N.Y.) of Sunday 27^th January 1952:

One of the imprints that launched the project was made by Tom Mix, first and perhaps greatest of cowboy stars. He brought his horse Tony to leave marks of the famous hoofs which “thundered” across the silent screen to piano music.
This was prophesied by what was earlier a great gag among his pint-sized fans:
Fan A. “Did you see Tom Mix?”
Fan B. “Tom Mix in what?”
Fan A. “Tom Mix’n Cement.”
(Laughter.)

This advertisement for Destry Rides Again (1932), Tom Mix’s first talking picture, was published in the Bradford Evening Star and the Bradford Daily Record (Bradford, Pennsylvania) of Thursday 2^nd June 1932:

 

The earliest occurrence of the phrase Tom Mix in Cement that I have found is from one of the unconnected paragraphs making up the column Side Lights, published in the Ithaca Journal-News (Ithaca, New York) of Wednesday 24^th December 1924:

J. Piffington Baze wants to know when Tom Mix in “Cement” is coming to our local movie emporiums.

The second-earliest occurrence of the phrase that I have found is from one of the jokes in As the College Humorist Sees the Campus, published in The Austin Statesman (Austin, Texas) of Sunday 21^st February 1926:

“Didjaever see Tom Mix in Cement?”
“No, but I’ve seen him in Disposed.”
—Bucknell Belle Hop.

The phrase then occurs in College Humor: The Best Comedy in America, by Collegiate World Publishing Co., published in several U.S. newspapers in November and December 1926—for example in the Chattanooga Daily Times (Chattanooga, Tennessee) of Sunday the 7^th November:

“What’s the trouble? You look sick.”
“I just saw Tom Mix in ‘Cement’ and it upset me.”
—Rutgers Chanticleer.

This advertisement was published in Simpson’s Daily Leader-Times (Kittanning, Pennsylvania) of Monday 19^th March 1928:

Tom Mix in Cement was some show. But this is some “Wow”
Dance from 9 to ?
Monday, March 19^th
at Eagles Hall
Also Pig and Spaghetti Roast from 10 to ?
Spaghetti Contest at 12:30
$2.50 per couple

 

The U.S. musician Erwin ‘Pappy’ Trester (1902-1975) used a variant of the phrase in an interview with Bob Junghans, published in the Winona Daily News (Winona, Minnesota) of Sunday 7^th August 1966:

The days of the big bands are over and Trester is the first to admit it.
“Everybody’s been hypnotized by the tube,” said the outspoken musician. “When people get home at night they kick off their shoes, open a bottle of old swamp water and watch Tom Mix and cement for the 10^th time rather than go out some place.”
And it’s television, or the “tube” as Pappy calls it, that he blames for the fade out of the big bands.

Edgar Williams wrote the following in his column The Scene In Philadelphia and its suburbs, published in The Philadelphia Inquirer (Philadelphia, Pennsylvania) of Saturday 11^th February 1989:

You have been around for quite a few winters if you ever […] answered “Tom Mix in Cement” when asked, “What’s playing?”

The phrase has occasionally been used in British and Irish English. For example, the following is from Of oof, oafs and unspoken oaths, by Hugh Leonard, published in the Sunday Independent (Dublin, County Dublin, Ireland) of Sunday 31^st March 1996:

One upon a time, the Pavilion splendidly resembled a great, white Mississippi stern-wheeler. Then it was gutted by fire, and the wags had a field day. “Guess what’s on in the ‘Pav’?” they would ask. Taken unawares, you would say “What?” only to be told either “Blue Skies Above” or “Tom Mix in ‘Cement’.”