"I believe that yesterday's events could become a turning point", Johnson wrote, adding, "The Western alliance took decisive action and Britain's partners came together against the Kremlin's reckless ambitions".
Britain and Russian Federation have expelled 23 of each other's diplomats since the March 4 attack, which the United Kingdom blames on Moscow.
Moscow has denied any involvement in the incident.
The White House announced Monday it had ordered the Russian diplomats to leave - a response to the poisoning this month of a former Kremlin spy and his adult daughter in a British shopping district.
"I welcome the worldwide support that we have garnered", she said.
Washington led the way in responding, ordering out 60 Russians in a new blow to US-Russia ties less than a week after President Donald Trump congratulated his Russian counterpart Vladimir Putin on his re-election.
On Monday, European Council President Donald Tusk announced in Varna, Bulgaria, that 14 European Union (EU) member states had made a decision to expel Russian diplomats in response to the poisoning of a former Russian spy and his daughter in Britain.
The Izvestia daily dismissed the expulsions as a "russophobic flashmob".
They remain in critical condition after exposure to a nerve agent, which, according to the British authorities, could have originated from Russian Federation.
May's spokesman said Tuesday that the "unprecedented" actions by allies were in part because they "recognize the threat that these Russian networks posed to the security of their own countries".
British Prime Minister Theresa May on Tuesday told UK Parliament that 23 countries had expelled more than 115 Russian intelligence diplomats, which "represents an unprecedented series of expulsions that demonstrates to the Kremlin that we will not tolerate their attempts to flout global law, undermine our values or threaten our security".
In Washington, U.S. Defense Secretary Jim Mattis told reporters that Russian Federation "has the potential to be a partner" with North Atlantic Treaty Organisation members but had "chosen to seek a different relationship".
"There was a time when this tactic of sowing doubt might have been effective, but no one is fooled any more".
It is also closing the Russian consulate in Seattle.