Ireland midfielder McPhail, who has spent seven years with the club and made over 200 appearances, will bid farewell to the Welsh capital when his contract expires at the end of June.
Also departing is Helguson, 35, who scored nine goals this season and was a regular in Mackay's starting line-up as they clinched promotion to the top flight.
Goalkeeper Elliott Parish and strikers Nat Jarvis and Jesse Darko will also leave this summer having not been offered new deals.
Long-serving right-back Kevin McNaughton has been offered a new deal as have youngsters Adedeji Oshilaja and Ronnie Hawkins while the Bluebirds took up an option on full-back Luke Coulson.
Six of Cardiff's apprentices have been offered contracts meaning Dane Griffiths, Gethyn Hill, Kane Owen, David Richards, Theo Wharton and Josh Yorwerth will turn professional this summer along with Tommy O'Sullivan who still had a year left as an apprentice.
- Sports & Recreation
- Jesse Darko