Georgia Jones, 18, had a “45-minute fit” and was taken to hospital after “taking two pills” at the Hampshire music event on Saturday.
The teenager and a 20-year-old Tommy Cowan both died, as organisers warned of a “bad batch” of drugs at the annual two-day gathering in Portsmouth.
Mr Cowan is said to have recently become a father.
Another person is in a critical condition and 12 others have been treated at the Queen Alexandra Hospital in Portsmouth after the first night of the Mutiny Festival.
A spokeswoman for the hospital said not all the admissions were “necessarily drugs-related”.
Georgia’s mother Janine Milburn has posted a plea on Facebook for people not to take drugs, adding that she hopes her message stops others from “ending up the same”.
Ms Milburn wrote on Facebook: “Georgia died yesterday due to complications after taking two pills at Mutiny.
“If nothing else I hope what has happened to her will deter you from taking anything ever.
“The pills had caused her temperature to rise so high it made her fit for 45 minutes.”
Ms Milburn added: “My little girl was 18 and full of life.
“I just hope this stops at least some of you from ending up the same.”
According to Portsmouth local paper The News, Mr Cowan’s father, Damian Cowan, 43, said: “He was funny. He loved life and he loved his little boy.
“He was everything. He was a good lad – unfortunately he made a bad choice.”
Mr Cowan was a “brilliant” friend, according to 19-year-old Jack Ford, who said they met at college studying uniformed services.
“He is such a caring guy who would do anything for any one. He loved going out with his mates but most importantly loved spending time with his girlfriend and son,” said Mr Ford, who lives near Portsmouth.
“It was a big shock to lose such a great guy like him.”
Both of those named as victims were from Havant, Hampshire.
Hampshire Constabulary are investigating the two deaths which they are treating as separate and non-suspicious.
Police dogs have been seen at the site of the annual two-day festival in King George V Playing Fields in Cosham.
Dizzee Rascal and the actor and DJ Idris Elba had taken to the stage on Saturday night.
Three more people were treated on Sunday at the hospital after suffering injuries due to assaults or because of alcohol-related intoxication, a Queen Alexandra spokeswoman added.
Mutiny Festival organisers warned at around 9.30pm, after the pair were taken ill, that it was “aware of a dangerous high-strength or bad batch substance on site” and that no drugs should be taken.
They also urged customers to “responsibly dispose” of any substances.
Craig David and Sean Paul were due to headline on the Sunday night before the day was cancelled.
The organisers said in a statement on Facebook afterwards: “The safety of our amazing customers has always been paramount to us and so to keep everyone safe and in respect to those who have passed, we have taken the decision not to open today.
“As you can imagine, this decision was not taken lightly and was taken with the support of the local statutory authorities who we continue to work with.
“Enquiries are being made into the circumstances of what has happened, but we must reiterate our advice to all our customers to responsibly dispose of any substances.”