It has been a monumental season for the pair having made it into the first-team at Chelsea from coming through to academy - a chance which has been hard to come by in recent years at Stamford Bridge.
But they are in their element. Going toe-to-toe with some of the best players in the Premier League.
But whilst they were at Derby County last season, they were blessed with big cup ties. Manchester United first in the Carabao Cup and Lampard, Mount and Tomori came through it and shone, and beat the Red Devils on penalties at Old Trafford.
The draw awaited to find out their next opponents, and out came Chelsea. A short wait for the Rams saw them be drawn out for a trip to Stamford Bridge.
Standard procedures don't usually allow loanees to play against their parent clubs if both sides meet, and Mason Mount and Fikayo Tomori thought that would be the case once again.
But Frank Lampard, a man loved and connected with every individual connected with Chelsea, pulled the strings for the young lads to play against the Blues in what would turn out a special homecoming for Frank Lampard. A real chance to say thank you for his efforts as a player at the club.
In an interview with the Daily Telegraph, the pair spoke on the draw and the feelings they felt at the prospect of playing at Stamford Bridge, but against Chelsea.
"It was mixed emotions because we’d come through the Chelsea academy and never played for them and then we were playing against them at Stamford Bridge. It was a weird feeling," said Mount.
Fikayo Tomori had a similar occasion with Hull City, whilst on loan at the KCOM, when Chelsea faced Hull, but it was a change in fortune for the 21-year-old: "I was very surprised. I was at Hull the year before with Michael Hector and Ola Aina when we got Chelsea in the FA Cup. I was like ‘imagine if we could play’ and the manager just said ‘nah’.
"So when we got Chelsea with Derby, I told Mason ‘listen, it’s not happening’. We had a day off and the manager [Frank Lampard] came back and said ‘I’ve been pulling strings for you and you’re allowed to play against Chelsea’. I was more surprised than Mason because in my head it wasn’t going to happen."
'Frank Lampard's Derby County' came to town and they held up a fight, but for Fikayo Tomori it all went wrong within just five minutes. The central defender turned it into his own net to give his parent club the lead.
"I just tried to get on the right side of the Chelsea fans nice and early! Seriously, though, after that I was thinking ‘this could only happen to me’," Tomori said on the own goal in front of the Shed End. "As soon as that half-time whistle went I was thinking ‘did that really just happen?’ It was such a weird one. There was so much going on in my head."
Now for the pair and Frank Lampard, they face Manchester United once again the Carabao Cup, this time with Chelsea. A tie which they are expected to win, and both will be looking to continue their form which has made them regular starters at Stamford Bridge this season.
You can follow live coverage of Chelsea versus Manchester United right here on SI.com.