"At times it felt like we would never cross the Brexit finish line, but we've done it", Johnson said after both houses of the British parliament ratified the withdrawal bill on Wednesday.
Despite Johnson's repeated promise to "get Brexit done" on January 31, the day of departure only marks the start of the country's European Union exit.
Charles Michel and Ursula von der Leyen put their signatures to the document in Brussels after the Queen gave royal assent to the legislation implementing the agreement on Thursday.
Boris Johnson has hailed crossing the "Brexit finish line" after the Parliament approved his Withdrawal Agreement Bill for the final time.More news: French tourists missing after Canada snowmobile crash
'Now we can put the rancour and division of the past three years behind us and focus on delivering a bright, exciting future - with better hospitals and schools, safer streets and opportunity spread to every corner of our country'.
In a keynote speech on 31 January, Prime Minister Boris Johnson is expected to pledge to "mobilise the full breadth" of the country's new freedoms following Brexit, according to a memo obtained by The Daily Mail.
The UK is set to enter a transition period in February, during which the relationship with Brussels will remain the same while trade talks are finalised before the tight December 31 deadline.
After January 31, the United Kingdom will enter a transition period ending on December 31 without any meaningful changes taking place in the relations between the United Kingdom and the EU.More news: Australian Open 2020: Fired-up Federer cruises into third round
Johnson returned as May's successor in July a year ago and has since managed to negotiate his own deal with Brussels and regain the government's control of parliament in a risky early election last month. Once passed, the conditions of the withdrawal bargain - about the UK's monetary obligations, taxpayers' rights, and also agreements for Northern Ireland - will probably have the power of a global treaty.
Johnson will celebrate his victory by issuing commemorative coins and chairing a special cabinet meeting in England's pro-Brexit north on January 31.
Speaking at the World Economic Forum in the Swiss ski resort of Davos, U.S. Treasury Secretary Steven Mnuchin said a deal with Britain was "an absolute priority of President Trump and we expect to complete that within this year". The U.K.is also keen to begin talks on a free trade deal with the United States, and intends to negotiate simultaneously with the European Union and the U.S.
U.S. Commerce Secretary Wilbur Ross said a trade deal between the two countries shouldn't be too hard because their economies are similar.More news: Caroline Wozniacki's tennis career comes to an end at Australian Open