The duo have been handed long-term contracts and will add experience to a young Chelsea side managed by former Arsenal Ladies assistant coach Emma Hayes.
It will be third time Chapman has worked with Hayes, after also playing under her for Chicago Red Stars in Women's Professional Soccer in America.
The midfielder, who made her debut for Millwall Lionesses at just 14 years of age, has also played for Fulham Ladies and Charlton Athletic Ladies and has represented England on 82 occasions.
Flaherty has just received her first international call-up and will team up with Chelsea days before jetting out to La Manga to join Mark Sampson's England training squad.
The 22-year-old defender scored the first-ever goal in the FAWSL back in April 2011, ironically netting the only goal of the game for Arsenal away at her new club Chelsea.
The former Arsenal pair, both originally from South London, take Chelsea's summer recruits to four, following the capture of former Birmingham City duo Laura Bassett and Rachel Williams.
Chapman, who is keen to help Chelsea improve on last season's position of second-bottom in the table, told Sky Sports News: "This is a new challenge for me and one I am looking forward to.
"We are building for the future and hopefully I can come here and prove myself as a player, and prove my worth to the team while helping make it stronger.
"We are aiming for higher things this season and hopefully I can play my part in that."
Flaherty admitted the lure of professinal football was one she could not turn down once she became aware of Chelsea's interest.
"It was a really tough decision for me to leave as Arsenal have been like a family to me," she said.
"I've been there since I was a youngster but like Katie said it's time for a new challenge for me, personally.
"I was working in three or four jobs over the past few seasons so for me to have the chance to come here and be a full-time professional was too good an opportunity for me to turn down."