The duo has finally been unveiled by the Premier League club after weeks of speculation, but will be back at the City Ground on loan for the season ahead.
The pair has signed for an undisclosed fee, believed to be in the region of 8 million euros and will be loaned back to the City Ground for the 2014-15 season.
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The pair have put pen to paper on seven-year deals at St James' Park, much to the annoyance of Forest manager Stuart Pearce, who hit out at owner Fawaz Al Hasawi for agreeing to the deal without his approval.
Lascelles, who has been capped at Under-20 level for England, and Darlow had both been courted by a host of Premier League clubs, including Tottenham and Manchester City, and spoke of their delight at signing for the north east outift.
"As soon as I found out about the move I was over the moon," Lascelles told the Newcastle United official website. "It's a huge step up for me. Newcastle are a club with a massive fanbase, a big history and an unbelievable stadium.
"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 in even better player."
Darlow, 23, also talked about his move, saying: "I'm delighted for it all to be sorted and I can't wait to get started here at Newcastle. The loan back is going to be another year but it gives me time to develop my game further before I return.
"I'm looking forward to the future."
Newcastle boss Alan Pardew had already described the pair as two "great assets" for the club when speaking to the press after his side's 2-2 draw with Huddersfield, and said he was delighted to get the deal over the line.
"It is a real coup to have signed two of the finest young talents in English football outside of the Premier League," said Pardew.
"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."