Guerin, a four-time all-star, scored 36 goals last season while splitting time with the St. Louis Blues and San Jose Sharks. He joins a team that suddenly is rebuilding after losing forwards Ryan Smyth, Jason Blake, Richard Zednik, Viktor Kozlov and defenceman Tom Poti since the free-agent signing period began last Sunday.
Comrie had 20 goals and 25 assists in 65 games with Ottawa, then added two goals and six points during the Senators' run to the Stanley Cup finals.
Despite the hits over the past few days, Islanders general manager Garth Snow vowed to be patient and prudent as he negotiated with players still on the market.
Financial terms weren't immediately available for either player.
"My family and I are so happy to be coming to Long Island," Guerin said in a statement. "Garth Snow first contacted me on Sunday and was so positive, so persistent right from the outset explaining to me why I would be ideal for his team. Ownership's commitment to winning at almost every cost and Garth making it clear to how determined he is to build a winner is why I'm an Islander today."
The Islanders lost their top two scorers from last season when Blake signed a five-year, US$20-million contract with the Toronto Maple Leafs, and Smyth spurned a hard push from New York and left for a five-year deal worth $31.25 million with the Colorado Avalanche.
Blake had team highs of 40 goals and 69 points, while Smyth - acquired from Edmonton for two former first-round picks and the Islanders No. 1 draft selection this year - finished second with 36 goals and 68 points.
The 36-year-old Guerin, a three-time U.S. Olympian, has 364 goals and 719 points in 1,026 career NHL games. He was chosen as an all-star for the fourth time last season when he put up 56 points in 77 games. He was dealt to San Jose at the trade deadline, but managed only two assists in nine post-season game for the Sharks, who were knocked out in the second round by Detroit.
He came up to the NHL with New Jersey, with whom he earned a Stanley Cup ring, and has also played with Edmonton, Boston and Dallas during 15 seasons.
"Bill Guerin, by everyone's account, is the consummate professional, the ultimate teammate," Snow said. "And beyond that, he's an outstanding hockey player coming off a 36-goal season. The Islanders are extremely proud that Bill has made the decision to come to Long Island."
In 386 NHL games with Edmonton, Philadelphia, Phoenix and Ottawa, the five-foot-10 Comrie has 123 goals and 262 points.
"We really wanted Mike Comrie because (coach) Ted Nolan, the scouts and I were unanimous in wanting to bring his offensive production, creativity and competitiveness to our team," Snow said. "At 26, Mike has already produced a lot of points and still has the potential to develop into one of the best centres in the game.
"I know Mike had a lot of options, but he understands the kind of team we're putting together on the Island and we're so glad he chose to be an Islander."
Guerin and Comrie are the third and fourth free agents to sign with the Islanders. Forward Jon Sim left Atlanta on Sunday for a three-year, $3 million deal, and former Tampa Bay left winger Ruslan Fedotenko signed a one-year deal Wednesday for $2.9-million.
Forward Trent Hunter and defenseman Chris Campoli are unsigned restricted free agents. Rugged right winger Arron Asham is unrestricted.