The strikes were conducted with the United States and France.
Britain's defense ministry said initial indications were that the precision weapons and meticulous target planning had "resulted in a successful attack".
May said the aim was to deter the Syrian authorities from further use of chemical weapons and to send a message to the wider world that it was unacceptable to use such weapons.
Scottish First Minister Nicola Sturgeon said Syria's use of chemical weapons could not be tolerated but questioned whether the strikes would halt their use or contribute to ending the civil war.
She added that the strikes were not about "regime change".
When asked if Syria's Assad could remain leader as long as he refrained from further use of chemical weapons, May said: "This was about, as I have said and you have recognized, this was specifically about the use of chemical weapons".
In her comments, May also alluded to a nerve agent attack in Britain last month on a former Russian spy and his daughter.
The Prime Minister is expected to face anger in the Commons after launching military action without securing the support of Parliament.
Trump defends 'mission accomplished' statement after strike on Syria
The airstrikes, which hit several sites, were a direct response to Assad's alleged use of chemical weapons in Douma on April 7. His choice of words recalled a similar claim associated with President George W Bush following the US-led invasion of Iraq.
"Very careful scientific analysis was applied to determine where best to target the Storm Shadows to maximise the destruction of the stockpiled chemicals and to minimise any risks of contamination to the surrounding area", the ministry said.
"I have done so because I judge this action to be in Britain's national interest", she added.
May is not obliged to win parliament's approval before ordering military action, but a non-binding constitutional convention to do so has been established since a 2003 vote on joining the US -led invasion of Iraq.
The Prime Minister will tell MPs on Monday that the strikes were in the national interest because the use of chemical weapons can not be normalised, including in the UK.
British Prime Minister Theresa May described the strike as "limited and targeted".
"Corbyn will rail against military action, claiming it could widen the conflict, but if he won't sanction military action against a regime that is using chemical weapons on its own people, when would he ever sanction it?" he told AFP.
He reiterated that Canada condemns the use of chemical weapons in Ghouta.