The couple, who reportedly welcomed their daughter on the morning of 16 December, are said to be very "emotional and excited" to have their first child together.
"Both mum and dad are over the moon; Christmas definitely came early!" a source told PEOPLE Magazine, exclusively .
The couple haven't yet confirmed the news on their social media channels or released their newborn daughter's name.
Rumours that Hashian was pregnant began to circulate earlier in 2015, but Johnson, who is also dad to 14-year-old Simone with his ex-partner Dany Garcia, only confirmed the news in November 2015.
"I was raised by and live with amazing and strong women, so the universe felt we needed one more... IT'S A BABY GIRL," he wrote on Instagram alongside a photo of him cradling Hashian's baby bump.
Hashian, 31, a singer/songwriter who has been with Johnson for the past nine years, took to Instagram on 12 December to gush over how excited she was to become a mum.
Uploading the same photo Johnson used in the baby announcement, she wrote: "To experience creating a little life is just incredible.
"This beautiful cycle of life and womanhood beginning again. Couldn't be prouder of all the kickass women she's going to have around her! And men of course."
In an interview with Esquire magazine in August 2015, Johnson said family life is just as important to him as his career.
"We talk so much about the business end, the success end, but then Lauren isn't mentioned and my daughter isn't mentioned," he said.
"I always like making sure we find the balance and my home life is in there and Lauren Hashian is in there and my daughter is in there."