Diadem was bred and owned by Edgar Vincent, 1st Viscount D'Abernon, and trained by George Lambton.

Diadem was a chestnut filly bred by Edgar Vincent, 1st Viscount D'Abernon and foaled in 1914.

The filly Sunny Jane took the lead of the race with four furlongs left to run and was then challenged by Diadem.

[8] In October she won the six-furlong Great Eastern Railway Handicap at Newmarket, beating Roscius by half a length.

[9] In May 1918, Diadem won the Salford Borough Handicap at Manchester by half a length from Golly Eyes.

[12] At Royal Ascot in 1919 Diadem started as the 7/2, fourth favourite for the King's Stand Stakes.

[15] At the Doncaster St. Leger meeting she carried top weight and started favourite for the seven-furlong Town Moor Handicap.

She took the lead with a quarter of a mile still to run and despite being challenged in the closing stages, she held on the win by a head from Quadrille.

[16] In October Diadem beat her only opponent, Freesia, by three quarters of a length to win the Challenge Stakes.

[26] In June 1921 Diadem finished fourth in the Royal Stakes at Epsom, behind winner Orby's Pride.