Speaking on the BBC’s Andrew Marr show, Mr Johnson said “enough is enough” as he referred to President al-Assad’s actions of abuse within his own country.
However, Theresa May has faced sharp criticism from other politicians for the UK’s role in the bombing, with Labour opposition leader Jeremy Corbyn questioning its legality.
Scotland’s First Minister Nicola Sturgeon has also warned that the air strikes “risk further escalating an already dangerous situation”.
Why did we bomb Syria?
The UK and US governments say the military assaults on Saturday were about preventing further chemical attacks by Assad’s forces on the Syrian people.
An alleged chemical attack killed up to 75 people – including many children – in the town of Douma on April 7.
Syria, backed by its main ally Russia, has repeatedly denied any involvement and said the bombing was a “violation of international law”.
Prime Minister Theresa May said the action “was focused on degrading and deterring the operational capability and the willingness of the Syrian regime to continue to use chemical weapons”.
She added: ”There have been many instances when we have seen them using those chemical weapons.
“But I believe it should also be a message to others that the international community is not going to stand by and allow chemical weapons to be used with impunity.”
The prime minister added the military assault was not about regime change, toppling Assad or intervening further in the war.
Although Western powers have participated in the seven-year Syrian conflict by arming rebels and bombing Islamic State fighters, they have previously refrained from attacking Assad’s forces – until now because of the alleged use of chemical weapons.
But US Ambassador to the United Nationals Nikki Haley has said her country is “locked and loaded” to strike Syria again if more such attacks continue.
How many missiles were fired at Syria?
Washington said the bombings were strategically targeted at three key sites where chemical weapons were being developed.
The areas destroyed included a research and development centre in Damascus plus two installations near the town of Homs.
A total of 105 missiles were fired in total by the allies.
According to officials, the UK’s Royal Air Force used four Tornado GR4s to attack a base 15 miles west of Homs.
The French launched 12 cruise missiles using Mirage and Rafale fighter jets while the US deployed B-1B Lancer bombers.