Brentford made 10 appearances last season, helping the Bees to promotion to the Premier League and the League Cup semi-finals. The 33-year-old was actually named in the competition's team of the tournament.
Even so, he was released at the end of the campaign and Boro have stepped in, with manager Neil Warnock impressed by the consistency of a player who has made more than 300 senior appearances.
"I've watched Luke for a few years now and I'm delighted to have him on board," he said.
"I've always found him very consistent in what he does. He's a good keeper, a good lad, and he'll be a great addition to our squad."
Daniels was a Manchester United youth product without playing senior football for the Red Devils. He moved to West Bromwich Albion but had loan spells with Motherwell, Shrewsbury Town, Tranmere Rovers, Charlton Athletic, Rochdale, Bristol Rovers and Southend United before finally making his Premier League debut for the Baggies in 2013. He later played for Scunthorpe United and Brentford.
Lumley, who spent part of last season on an emergency loan at Doncaster Rovers, joined earlier in the summer after being released by Queens Park Rangers.