"Entirely the right thing to do".
The air strikes followed a suspected chemical attack on civilians by the Syrian regime in Douma, east of Damascus, on April 7, in which 60 people were killed.
"In degrading Assad's chemical weapons capabilities we intend to do what we can to protect his people from that specific form of cruelty", he said.
"The erosion of that taboo, that has been in place for 100 years, has gone too far under Bashar al-Assad and it was time that we said "no" and I think it was totally, therefore, the right thing to do".
Mr Johnson said action had to be taken against Syria for the Douma attack during an appearance on The Andrew Marr Show on Sunday.
They will add: "The Council is supportive of all efforts aimed at the prevention of the use of chemical weapons".
In the early hours of Saturday morning, joint US, British and French forces launched a series of air strikes targeting what the Pentagon said were three chemical weapons facilities, including a research and development centre in Damascus and two other sites near Homs.
Prime Minister Theresa May is set to face British lawmakers to explain her decision to launch airstrikes against Syria without a vote in Parliament.
Mr Johnson said Mrs May will be making a statement in the House of Commons on Monday and it will give parliamentarians a chance to hold the executive to account.
"... We can not allow the use of chemical weapons to become normalised - either within Syria, on the streets of the United Kingdom or elsewhere".
Britain sought "to alleviate further humanitarian suffering caused by chemical weapons".
May was to tell parliament, "We have acted because it is in our national interest to do so".