Forest manager Stuart Pearce has not disguised his disappointment at losing two of the club’s brightest young players since the deal was agreed.
Lascelles and Darlow will however both be loaned back to the Championship club for the season and Newcastle boss Alan Pardew is delighted to have signed two promising players.
"It is a real coup to have signed two of the finest young talents in English football outside of the Premier League,” he told Newcastle’s official website.
"Jamaal and Karl will be a big part of the future of this club.
“They have both performed outstandingly in the Championship over the last couple of years and I have no doubt they will be able step up to this level once they return from their loan at Nottingham Forest."
Centre-back Lascelles came through the youth system at Forest and after a spell on loan spell at Stevenage, established himself as a regular at the City Ground last season.
"It's a huge step up for me. Newcastle are a club with a massive fanbase, a big history and an unbelievable stadium,” said the 20-year-old.
"First, I am going back to Forest for another season and that will be great for my development. Hopefully I can come here next summer an even better player."
Goalkeeper Darlow, another product of the Forest Academy, made his debut as a substitute on the final day of the 2010-11 campaign.
The 23-year-old has also enjoyed loan spells at Newport County and Walsall - twice – before establishing himself as Forest's first choice keeper since January 2013.
"I'm delighted for it all to be sorted and I can't wait to get started here at Newcastle," he said.
"The loan back is going to be another year but it gives me time to develop my game further before I return.