The veteran Blues man struck in the 19th minute against his old side with a well-placed header to put the hosts on course for their 2-0 win at Stamford Bridge.
Lampard was happy to reach the milestone and credited team-mate Eden Hazard, who provided the assist before scoring himself in the second half.
"It's special. It's obviously a landmark I've been getting towards," he told Sky Sports.
"I'm just pleased to get there and pleased it's an important goal for the team.
"I've been very fortunate to play for the club for a long time here with great players and players who put the ball on a plate sometimes for you like Hazard did there so I'm thankful.
"I think he's got the world at his feet and he can handle that pressure because he's that good."
Despite reaching the landmark, Lampard was frustrated not to make it 201 when he uncharacteristically blasted a chance over the crossbar in the second half.
He added: "I'm really annoyed. I won't sleep tonight.
"I love scoring goals and I love being able to arrive in the box. I think I relaxed a bit on that one and skied it over the bar."
Chelsea dominated against the Hammers, as they carried on the momentum from the second half of the FA Cup quarter-final with Manchester United, which they drew to force a replay on Easter Monday, and the Europa League defeat of Steaua Bucharest.
"The good thing is we have had three very good games, but the bad thing is we have the international break now," manager Rafa Benitez said.
"Hopefully, mentally, they will relax, they will change their environment and they will come back with a fresh mind and fresh legs."
Meanwhile, the Metropolitan Police are investigating after missiles were hurled from the away end in the direction of Chelsea's players as they celebrated Lampard's goal.
The incident could lead the Football Association to take disciplinary action.
CCTV footage will be studied to identify the offenders.