The former Friends star kissed and snuggled up to her boyfriend in the back of a cab after cheering him on from the venue's upper level on Friday night.
The following day, she continued to play groupie, watching the musician perform in London's Hyde Park.
Groupie: Jennifer Aniston cosies up to rocker John Mayer after his performance at the Brixton Academy on Friday night
Hands on: Jen dives in for a cuddle
She took in his act from the side of the stage, while busily taking photographs of his show for her own collection.
An onlooker said: 'Jennifer must have taken about 10 photos during Mayer's set, and she would look at each one afterwards.'
'She was definitely into the music. Her foot was tapping and her body was swaying.'
Happy snap: Jennifer photographs boyfriend John Mayer from the side of the stage in Hyde Park on Saturday
In the frame: The actress keeps focused on her man
Sheryl Crow, a close friend of Jennifer, also performed at the concert.
Of her Hollywood friend's budding romance, Crow said:'If she's happy, then I'm happy!'
Jennifer has been in the UK for the last week after she flew in to meet John as part of his UK tour, but has since returned to the US.
This one's for you Jen: John performs on the main stage at Hard Rock Calling in Hyde Park
The couple, who have been dating for three months, have attempted to keep out of the spotlight but it now appears their relationship is becoming more serious.
At the beginning of the month, he was introduced to Jen's best pal Courtney Cox and husband David Arquette.
Mayer, 30, has dated a string of Hollywood stars - including Jennifer Love Hewitt and Jessica Simpson - while Aniston, who was married to Brad Pitt for four and a half years, has been single since splitting from Vince Vaughn in December 2006.
Sheryl Crow - a good friend of Jennifer's - also performed
The view of the main stage during day one of the Hyde Park festival
