Lance received his entire education in Matamata, beginning at St. Joseph's Convent and continuing at the Intermediate School. He then attended Matamata College for three years before deciding to pursue a career that would utilise his equine skills rather than academic studies. Lance O’Sullivan’s first ride was on 12 June 1980 when he rode to win the Arapuni Handicap at Te Awamutu, a horse trained by his father.

O'Sullivan retired from riding in 2003 with a record 2358 New Zealand winners. In addition to this total he rode a further 121 winners offshore in places as diverse as Australia, Hong Kong, Japan, Macau, Singapore and Turkey. When he retired O'Sullivan was credited with winning: 12 New Zealand Jockey's Premierships (a record), having broken Bill Broughton's long-standing record of 11 and 62 G1 winners. O'Sullivan won most of New Zealand's feature races over his career, including the prestigious sprint Telegraph Handicap seven times and the Railway Stakes five times. Out of the 28 win on Australian soil, 10 were at Group 1 level. He won the 1992 Queensland Derby and Oaks on Royal Magic, the 1984 AJC Oaks on La Souvronne (the 2nd apprentice since WWII) and the 1991 Galaxy on Mr Tiz in remarkable fashion.

Near the back of the field at the home turn, and seemingly in a hopeless position with 300 metres to run, the incredible sprinter Mr Tiz showed a breathtaking turn of foot and made up an extraordinary amount of ground to record one of the greatest wins on Australian soil. Second last with a gap behind a wall of horses, Lance booted him along the rails but there was zero room to move - he was trapped. He managed to punch his way wider and finally find space on the outside very late, before powering up and absolutely storming home. It was potentially one of the finest sprint wins by any Kiwi horse to have ever crossed the ditch.

His biggest win as a jockey was the 1989 Japan Cup on champion mare Horlicks, breaking the world record for 2400m. When the local hero Oguri Cap battled Horlicks up the straight in the 1989 Japan Cup (2400m), 140,000 fans responded with a noise so loud jockey Lance O’Sullivan described it as “like a physical assault.” The pair pulled clear in an absolute dogfight, met by the roars of the masses at Fuchu. In Lance O’Sullivan’s biography Tears in the Wind, he described feeling the crowd noise as “vibrations through my entire body.” Horlicks time of 2 minutes 22.5 seconds broke Hawkster’s record In a remarkable and emotionally charged triumph. Lance also won the 1989 LKS Mackinnon on Horlicks in 1989.

Despite a number of attempts O'Sullivan was never quite able to win the Race That Stops The Nation: the Melbourne Cup. He came agonisingly close in 1985 when run down in the final few strides on Koiro Corrie May by What A Nuisance. He was also jockey of Waverley Star, who was unfortunate to run into the champion Bonecrusher, in the 1986 Cox Plate. Dubbed the "Race of the Century" Waverley Star finished a gallant second, after a two horse war with Bonecrusher from the 800m, only succumbing to the champion in the last few strides. The 1986 W. S. Cox Plate is one of the most replayed versions of the race and one of the most remembered horse races in Australian and New Zealand racing history. Our Waverley Star did prove victorious in the 1987 Chipping Norton Stakes.

He did achieve redemption, courtesy of Surfers Paradise, the previous seasons New Zealand Derby winner, who looped the field on the home turn to win the 1991 running of the Cox Plate. Lance O’Sullivan had no joy in his first three Cox Plate attempts, finishing 2nd in 1986 on Our Waverley Star in the "Race of the Century" against Bonecrusher, 11th in 1989 on The Phantom and eighth in 1990 on Horlicks, who a year earlier had won the Gr.1 Japan Cup (2400m) in world record time. But finally O’Sullivan’s luck changed in 1991 when he rode the outstanding galloper Surfers Paradise, trained by his father and brother. Entering Moonee Valley's short home straight, an audible roar breaks out from the crowd as Super Impose raced to the lead, and died away to silence as Surfers Paradise takes over midway down the straight. The trainer of the runner-up, Lee Freedman, quipped that it was the first time anything had ever come from behind to beat Super Impose. The pair also combined to win the 1991 Rosehill Guineas.

It wasn't always plain sailing though. In 1997 Lance suffered a horrific fall on Cox Plate day that shattered his leg. The injury was so servere after his mount Centie Lass crashed through the running rail, that his bone was sticking out and he thought his leg might fall off while being moved. He had to overcome a morphine addiction, sickening drug hangovers and two years of physiotherapy. Despite the serious injury keeping him off the scene for 18 months, O’Sullivan’s determination led to his return to gather more success, but he never got the opportunity to add to his Cox Plate record.

In the 2003 New Year Honours, O'Sullivan was appointed an Officer of the New Zealand Order of Merit, for services to thoroughbred racing. In 2006, O'Sullivan was part of the inaugural class inducted in the New Zealand Racing Hall of Fame. Lance O'Sullivan's father, Dave, founded the Wexford Stables at Matamata. Lance's brother, Paul, trained in partnership with their father before moving to Hong Kong Jockey Club in 2004. Starting in the 2006–07 season Lance O'Sullivan has trained in partnership with Andrew Scott who previously worked in partnership with Mike Moroney at Ballymore Stables. In recent times the duo have trained the outstanding three-year-old Molly Bloom, winner of the 2023 New Zealand 1000 Guineas, and stayer Asterix - winner of the 2022 New Zealand Derby and 2024 Auckland Cup.

New Zealand’s most famous and successful jockey broke all the records and finally secured his Cox Plate