"For those two, it was very much a love at first sight situation. They spend every waking second together. They are always together for each other's professional and personal events," the insider added.
The source noted that most of the time, Fox joins her man if he's going somewhere.
The brunette beauty explained in her Instagram announcement that while they've gone through their share of struggles and pain in "such a short, frenetic period of time," when he asked her to marry him, she said yes "just as in every lifetime before this one, and as in every lifetime that will follow it."
The "my ex's best friend" singer showcased his new fiancée's engagement ring on Instagram. He explained that he designed the ring with Stephen Webster to include her birthstone, which is an emerald, and his, a diamond. The jewels are "set on two magnetic bands of thorns that draw together as two halves of the same soul forming the obscure heart that is our love," he added.
Green has since been linked to Sharna Burgess. The source noted that he is "super into" the Dancing With the Stars pro.
Despite jumping into the whirlwind relationship so soon after her divorce, sources revealed the 35-year-old was "thrilled" about the proposal, and is "ready to move on completely" from her previous marriage.
It is not yet clear when Fox and MGK plan on tying the knot.