He was from the first crop of foals sired by Be My Guest, an American-bred stallion who won the Waterford Crystal Mile when trained in Ireland by Vincent O'Brien.

Two weeks later, Assert was moved up in distance for the Gallinule Stakes over 1+1⁄2 miles at the Curragh and recorded a wide-margin victory over Rivellino.

[4] Golden Fleece was Sangster's representative in the Epsom Derby and so Assert was sent to contest the French equivalent, the Prix du Jockey Club over 2400 m at Chantilly Racecourse four days later.

[5] Three weeks later, Assert started 4/7 favourite for the Irish Derby at the Curragh, with the American-bred, British-trained Silver Hawk appearing to provide his only serious opposition.

[6] Assert was then matched against older horses for the first time in Britain's most prestigious weight-for-age race, the King George VI and Queen Elizabeth Stakes over 1+1⁄2 miles at Ascot Racecourse on 24 July.

Assert went past Bikala on the turn into the straight and held off a strong challenge from Glint of Gold, but was caught inside the final furlong and beaten a neck by Kalaglow.

On 17 August Assert was ridden by Pat Eddery in the eleventh running of the Benson and Hedges Gold Cup over 10+1⁄2 furlongs at York Racecourse.

[4] At the end of his three-year-old season, Assert was syndicated with an estimated value of $25,000,000 and sent to North America to become a breeding stallion at the Windfield Farm in Maryland.

He sired many good winners, including Dancehall (Grand Prix de Paris), Lecroix (Union-Rennen), Tzar Rodney (Prix La Force), Timely Assertion (Santa Anita Oaks), Square Cut (San Luis Obispo Handicap), All My Dreams (Deutsches Derby), Betty Lobelia (Miss Grillo Stakes), Nomadic Way (Stayers' Hurdle, Cesarewitch Handicap, Irish Champion Hurdle) and Running Flame (Hollywood Turf Cup).