The US used about double the number of weapons that US forces deployed during last year's strike on Shayrat airfield, Mattis said.
"This is the first time as prime minister that I have had to take the decision to commit our armed forces in combat -- and it is not a decision I have taken lightly".
"Theresa May should have sought parliamentary approval, not trailed after Donald Trump".
The prime minister added the military assault was not about regime change, toppling Assad or intervening further in the war.
"The UK is permitted under global law, on an exceptional basis, to take measures in order to alleviate overwhelming humanitarian suffering", the statement said.
"We can not allow the use of chemical weapons to become normalized - within Syria, on the streets of the United Kingdom, or anywhere else in our world".
Parliament is not due to reconvene until Monday, following its Easter recess.More news: Kingdom Hearts 3 trailer shows off 80's LCD games-inspired mini-games
The group said it "strongly condemned" the action and accused May of "sanctioning killing" at Trump's behest.
The Prime Minister is expected to face angry MPs after launching military action without securing the support of the Commons.
David Cameron, who was prime minister in 2013, tweeted on Saturday: "As we have seen in the past, inaction has its consequences".
The United States is preparing to impose sanctions on Russian Federation for "covering up" the actions of the Assad regime.
May has repeatedly said that the missile strike on Syria was not about "regime change". The strikes at 0100 GMT were 15 miles (24 kilometres) west of Homs.
But she will say the UK joined the United States and France in co-ordinated strikes following the chemical weapons attack in Douma to "alleviate further humanitarian suffering".
"It is about a limited and targeted strike that does not further escalate tensions in the region and that does everything possible to prevent civilian casualties", she said.More news: IMD to issue forecast for 2018 southwest monsoon rains today
A YouGov poll for The Times newspaper this week indicated that only a fifth of voters believed that Britain should launch attacks on Syrian military targets and more than two-fifths opposed action.
Vince Cable, leader of the Liberal Democrats, the fourth-biggest party in parliament, accused May of "riding the coat-tails of an erratic U.S. president".
The centre-right Conservatives rely on the support of the Democratic Unionist Party, Northern Ireland's biggest party, for a majority in parliament.
Following the air strikes, there was criticism from some opposition MPs.
Tom Tugendhat, the Conservative chairman of parliament's foreign affairs scrutiny committee and a former army officer, said May had "taken the correct decision".
"But I believe it should also be a message to others that the worldwide community is not going to stand by and allow chemical weapons to be used with impunity".More news: Flood warning for Ulster; flash flood watches for Sullivan and Orange