A relatively successful jump jockey, Nicholls has become the leading National Hunt trainer of his generation in Britain, finishing the 2007–08 season with 155 winners and a record £4 million in prize money.

As of April 2023, he has trained over 3,500 winners, won the 2012 Grand National, four Cheltenham Gold Cups and has been crowned British jump racing Champion Trainer fourteen times.

The pair enjoyed numerous big race successes, including back-to-back wins in the Hennessy Gold Cup at Newbury with Broadheath in 1986 and Playschool the following year.

[1] In response to an advert placed in the Sporting Life, Nicholls began his training operation at Manor Farm, Ditcheat, in stables rented from local dairy farmer Paul Barber.

Nicholls enjoyed a memorable meeting, collecting the Queen Mother Champion Chase with Call Equiname,[5] the Arkle Challenge Trophy with Flagship Uberalles,[5] and the Gold Cup with See More Business.

[10] Nicholls' finest hour came in the 2008 Cheltenham Gold Cup where he saddled the first three horses home: in finishing order, Denman (ridden by Sam Thomas), Kauto Star and Neptune Collonges.

[11] At the high of his stable stars' powers, Nicholls' dominance extended across the Irish Sea and his horses plundered Ireland's most valuable jumping prizes with increasing regularity.