About 50,000 people were thought to have attended the concert, which comes 13 days after a suicide bombing hit Grande's show at Manchester arena, killing 22 people.
The concert at Old Trafford in Manchester City aims to support the families and victims affected by that attack.
Grande herself headlined the performance, and supporting acts include Justin Bieber, Coldplay, Katy Perry, Take That, Pharrell Williams, Niall Horan, Miley Cyrus, Usher, Robbie Williams, Little Mix and The Black Eyed Peas.
Everyone who attended the Manchester Arena concert was invited to apply for free tickets to the event.
Grande said she wanted to return to the "incredibly brave city" to spend time with her fans and to "honour and raise money for the victims and their families".
The night before the concert was due to take place, London was [hit by a terror attack at London Bridge], which left seven people dead and 48 injured.
Grande tweeted after the attack in the capital that she was "praying for London".
After the attack, her manager Scooter Braun said the concert would go on as planned. He called the event a "statement that hatred and fear will never win".
"Today's One Love Manchester benefit concert will not only continue, but will do so with greater purpose," Braun said in a statement.
"We must not be afraid and in tribute to all those affected here and around the world, we will bring our voices together and sing loudly."
The Bishop of Manchester, the Right Reverend David Walker, led a minute's silence ahead of the music at the concert.
"So I ask you now to fall silent and for this next minute to remember those who died, remember those who were injured, remember those who were bereaved in Manchester and in London last night," he said.
After the minute, he told those gathered to "choose love".
Grande teamed up with fellow pop star Miley Cyrus to deliver an emotional rendition of the Crowded House classic Don't Dream It's Over.
Grande, 23, initially performed Be Alright and Break Free.
Police said additional security was in place, with armed officers stationed around the ground.
BBC Radio 5 Live's entertainment correspondent Colin Paterson outside Old Trafford, and said there had been a notable increase in security, with a "ring of steel" built around the venue, police officers brought in from all over the country, and armed police at road junctions and gates.
Fans said they wanted to be there both as an act of solidarity to Ariana Grande and to not let terrorists win, he said.
Proceeds from the event will go to the We Love Manchester Emergency Fund which was set up by the Manchester City Council in partnership with the British Red Cross.
The fund was set up to support those affected by the suicide bombing at Grande's concert at Manchester Arena on 22 May, which left 22 people dead.