The appointment follows the news that Sam Ashton-Booth had stepped down as head chef of the restaurant last week to focus on “a few personal things”.
Millar was formerly sous chef at Stem, with his CV also including stints as a chef de partie at The Ledbury in Notting Hill and The Vineyard at Stockcross.
"It is fantastic to have the opportunity to promote someone who has been with me since Stem opened, and who shares the same vision for the restaurant as I do,” says Jarvis, who is also chef patron of Farringdon’s Anglo restaurant.
“Last week we launched a new menu which Tom and I have been working on together.”
No official reason was given for Ashton-Booth’s departure, but in a statement on his Instagram account the chef said: “I am proud of everything we have achieved here [at Stem], and would like to thank all the suppliers, staff and guests for the last 12 months.
Stem launched in January 2018 as part of a regeneration of the area by landlord The Crown Estate. It was Jarvis’ second restaurant to open in the area around London Oxford Street and Regent Street, following on from the opening of Neo Bistro on Woodstock Street, which is now closed.