Albert Buttigieg will be filling in the seat vacated by the death of Nationalist MP Robert Arrigo in the 10th district.
The St Julian's mayor got elected with 2,122 votes, with the other contender, lawyer Emma Portelli Bonnici getting 1,151 votes.
The counting process, with electronic counting, was over within seconds.
The other potentially major contender, former MP Karl Gouder did not contest.
Following Arrigo's death, there was speculation in PN circles that none of the candidates would apply, making co-option necessary. This would have potentially allowed Alex Perici Calascione, the party's deputy leader, to be nominated to parliament.
Both Buttigieg and Portelli Bonnici said they were submitting their names for the casual election out of respect for the democratic process and the citizens in the 10th district.
Speaking after he was elected on Friday, Buttigieg denied facing any pressure to not contest the casual election.
In his first comments after being elected an MP, Buttigieg paid tribute to Arrigo.
He said it was a big responsibility to fill his seat and he planned on continuing to be close to the people and their issues, acting as a servant of the common good.
Buttigieg had clashed with PN leadership in the weeks after the general election held earlier this year, saying that he had been intentionally kept away from the limelight and attributing that decision to his willingness to criticise so-called "fat cats".
In a Times of Malta opinion piece published last May, Buttigieg wrote that an acquaintance had overheard a top PN official promise one such "fat cat" that the party would "shut me up".
The claims triggered an internal party probe.
Asked on Friday how he will work with those he had called fat cats, Buttigieg made it clear that he considered the matter to be in the past.
"I have a pair of eyes to look forward rather than back," he said, adding that he looked forward to working with his colleague MPs under the direction of party leader Bernard Grech to bring about positive changes in the country.
In a post on Facebook, Grech said he welcomed Buttigieg at his office to congratulate him and discuss the work of the party's parliamentary group.
Meanwhile, Portelli Bonnici told her Facebook followers that while this was not the result they wanted it was one they expected.
"I wish Albert Buttigieg all the luck in his new role - we have more in common than one might think. I'm glad I took the step to submit my nomination for these casual elections, as it helped build some bridges between myself and Partit Nazzjonalista as well as with the electorate.
"As always, I remain available to the party and to anyone who might wish to reach out. As originally promised, I'm just getting started," she said.
Correction October 28, 2022: A previous version misquoted Buttigieg in his reply to a question. It was the journalist, not Buttigieg, who used the term "fat cats".