5 July 1932, San Francisco Chronicle: “Be kind to everyone on the way up; you’ll meet the same people on the way down.” Credited to “Wilson Miznor.”
9 July 1932, Walter Winchell’s column: “Always be pleasant to the people you meet on the way going up—for they are the same people you meet on the way down!” Explicitly called “Wilson Mizner’s classic advice.”
June 1933, Reader’s Digest: “It pays to be nice to the people you meet on the way up, for they are the same people you meet on the way down.” Credited to Winchell, not Mizner — the version that spread furthest fastest.
10 October 1942, The New Yorker: “Be nice to people on your way up because you’ll meet ’em on your way down.” The earliest known appearance of the wording modern quote sites recycle, and it is back under Mizner’s name.
1952 and 1958, quotation anthologies: “Be awful nice to ’em goin’ up, because you’re gonna meet ’em all comin’ down” — printed in Durante’s stage dialect and credited to him, in Franklin Pierce Adams’s FPA Book of Quotations and Herbert V. Prochnow’s The New Speaker’s Treasury of Wit and Wisdom. This is where the Durante attribution hardens.