The 19-year-old replaces Sam Twiston-Davies who has chosen to go freelance and focus on riding for his father Nigel.
The announcement precedes the start of the new National Hunt season which begins at Uttoxeter and Hexham today just a week after the 2017-18 campaign drew to a close.
Picked by Nicholls to succeed Ruby Walsh, Twiston-Davies fell out of favour during the past season – and then opted to ride his father’s Blaklion in the Grand National at Aintree.
This coincided with Nicholls giving more opportunities to Bryony Frost, Sean Bowen and, notably, Cobden who partnered Diego Du Charmil to a Grade One success at Aintree on Grand National day.
Yet Nicholls will have been aware that Cobden, the former champion conditional, was being used with increased frequency by his Westcountry rival Colin Tizzard – they teamed up to win the Albert Bartlett Hurdle at the Cheltenham Festival with Kilbricken Storm.
Cobden then produced one of the rides of the season on Tizzard’s Ultragold in the Topham Chase over the National fences when he took a pull to ensure he was not taken wide by a riderless horse.
That manoeuvre saw Cobden named jockey of the month for April ahead of Davy Russell who won the National itself on Tiger Roll.
“Harry will become our number one as Sam is going freelance. I expect Sam to have as many rides and winners for me over the next 12 months as he did during the last season,” said Nicholls. “Harry has been with us since the age of 16. A lot of my owners have wanted to use him. We’ve lots of good jockeys here and it’s hard to keep everybody happy.
“Sam can ride for everyone now. There has been no falling out. We’re still all mates, it’s just a restructuring on both sides.”