Mount has yet to make his debut for Chelsea's first team but impressed for Championship side Derby during the last campaign, scoring 11 goals in 44 appearances in all competitions.
The midfielder follows Ruben Loftus-Cheek in signing a deal through to the summer of 2024, with his new terms understood to be worth around £75,000 a week.
On signing his new deal, the England U21 worldwide said: 'It's a massively proud moment for myself and my family.
Lampard, on the other hand, is a Chelsea legend, having finished his career as the club's all-time top goalscorer, winning 13 trophies during his 13-year spell at Stamford Bridge.
Chelsea have also completed the signing of Croatian midfielder Mateo Kovacic from Real Madrid after a season-long loan.
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The Magpies have sold Ayoze Perez to Leicester and his strike partner from last season, Salomon Rondon is back at West Brom. Bruce confirmed talks with Newcastle had taken place over the weekend and a deal isn't far off.
Chelsea have been banned from making new signings for the next two transfer windows by Federation Internationale de Football Association, following investigations into their purchases of foreign Under-18 players.
"It's great to have him back and not just him but all his staff as well because they have known this club for a long time, they love this club and they were waiting for this opportunity".
As a result of their transfer ban, Lampard will likely have to look to Chelsea's vast youth reserves to help support his squad. "We need to work a lot and understand what we have to do". "The young players are incredible, they have the quality already to play for this club, so I have to do my role as a senior player trying to help them and also doing my job".
Therefore, Chelsea academy players may play a big role in their season, a situation which, according to Giroud, Lampard can manage.
"Now we have a batch of them that are fighting to get into the first team", Lampard said of his young players.