The Republican senator and former presidential nominee in the 2012 election announced he would not seek a second term in the Senate.
Mr Romney, who also served as the governor of Massachusetts, has held the seat since 2018 and said in a video message that at the age of 76 he did not think he would be capable of continuing in the role.
In response to the news that one of his most vocal critics was stepping away from politics, the former president took to his social media platform Truth Social in celebration.
In an all-caps post, he wrote: “Fantastic news for America, the great state of Utah, & for the Republican Party.”
“Mitt Romney, sometimes referred to as Pierre Delecto, will not be seeking a second term in the US Senate, where he did not serve with distinction,” he continued.
“A big primary fight against him was in the offing, but now that will not be necessary. Congrats to all. Make America Great Again!”
The reference to “Pierre Delecto” is from when Mr Romney was outed as the owner of an anonymous Twitter account under that pseudonym.
In an interview with USA Today in 2019, the senator said the account had been set up by his son Matt in 2011 and he had not given much thought to the name. Mr Romney said he mostly used it for following news.
Mr Trump, on the other hand, has been known in the past to pretend to be his own publicist on the phone — the fictional John Barron or John Miller.
Senator Romney, appearing as himself, said in his video statement: “I spent my last 25 years in public service of one kind or another. At the end of another term, I’d be in my mid-80s. Frankly, it’s time for a new generation of leaders. They’re the ones that need to make the decisions that will shape the world they will be living in.”
Those words are especially poignant as senators from both parties face questions about their advanced age and whether it is Mr Trump or a re-elect President Joe Biden taking the oath of office in January 2025, they would be 78 and 82 respectively.
Senator Dianne Feinstein, the oldest serving member of the body at 89, has faced frequent questions about her declining memory.
Similarly, Senate Minority Leader Mitch McConnell, 81, faced two episodes where he froze when speaking to reporters both in Washington and Kentucky.
During his time in the US Senate, Mr Romney emerged as a dealmaker and intermediary between Democrats and Republicans, brokering the first two rounds of Covid-19 relief funds during the Trump administration.
In addition, Mr Romney emerged as one of the few vocal critics of Mr Trump during his presidency, serving as the sole Republican who voted to convict Mr Trump during his first impeachment in 2020 and one of seven Republicans who voted to convict Mr Trump in his second impeachment for his role in the January 6 riot.
During the Biden administration, he negotiated the bipartisan infrastructure bill, the first piece of gun safety in almost 30 years and a bill codifying protections for same-sex and interracially married couples. He also voted to confirm Justice Ketanji Brown Jackson to the Supreme Court.
In his unsuccessful run for the presidency in 2012 he lost to incumbent President Barack Obama, with the Democrat winning 332 Electoral College votes to his 206, and 66 million votes to his 61 million.