John Phillips was one of the founding members of The Mamas & Papas and died of heart failure in 2001. Being a huge influence on harmonies of the time, The Cowsills were doubtlessly influenced by the beautiful sounds of The Mamas & Papas. And we can't forget that The Cowsills recorded "Monday, Monday" on their In Concert LP.
John Cowsill was also the drummer for a version of the M&P's in the 80's. There is audio of John playing with John, Denny Doherty, MacKenzie Phillips, and Spanky McFarlane.
One fun story involving the two John's is this. Doug writes: "I arrived late in the afternoon and Kevin and I checked into the Wyndham. This was a brand new hotel and I don't think it was even open to the public yet. AFIK the talent for this show and us were the only guests staying there. As we were checking in we ran into John Cowsill. The three of us ended up in Kevin's room partying for a while before the show. We were singing and John gave me a look that could have killed. You see there was a Beatle song playing and John and I were singing harmonies with it. He, like many of us, is a Beatle nut. The problem is that I sang a non Beatle harmony which fit but was not proper. He told me that was sacrilegious and I guess he was right. :D
... The finale was the Mama's and the Papa's. This incarnation had John, Denny, Mackenzie Phillips, and Spanky McFarlane. John Cowsill was playing drums. He was very stiff and nervous but definitely competent. He seemed somewhat intimidated by John Phillips. Remember that when the Cowsills had their hits John was around 10 years old and the Mama's and Papa's were on the charts at that same time and John Phillips was pretty big time. I was feeling a bit guilty for all the partying we had done before the show, but after all it was New Year's Eve. I sure didn't want any responsibility for making John any more uncomfortable than he was.