I only mentioned Robbie Robertson because he's of the 70's (Born July 5, 1943 (age 73))

Me.
 
;-)

Mason Williams. 'Classical Gas' was the first tune that made me want to pick up a guitar

There were no under-rated guitarists in the 70's.  Only some that were over-rated that made the others appear under-rated by comparison.
 
HTH.
 

 
(I guess I'll add Steve Bartek & Walter Trout in the U.S. anyway.)

lol. I was thinking of Ronnie Montrose and Frank Marino. When I went to look up if Frank was still alive, wikipedia showed this.
 
"Francesco Antonio "Frank" Marino (born November 20, 1954) is an Italian Canadian[2] guitarist, leader of Canadian hard rock band Mahogany Rush. Often compared to Jimi Hendrix, he is acknowledged as one of the best and most underrated[3] guitarists of the 1970s."
 
Sometimes you just can't make this stuff up.
 
Unfortunately, Ronnie Montrose passed a few years ago. RIP.

Yes, definitely Frank Marino - Kim Mitchell is another who comes to mind. 

Two who are no longer with us -

Kelly Johnson (Girlschool, whose drummer Denise Dufort also deserves far more recognition than she gets).
Huw Lloyd-Langton (Hawkwind, Leo Sayer, Widowmaker, Lloyd-Langton Group).

On the plus side Wilko Johnson is at last getting the recognition he deserves, and in his case unlike many who only get recognised until after they die he was told he had terminal pancreatic cancer and organised an urgent farewell tour only to go through a roller-coaster couple of years ending in a huge and experimental operation and medical process with a survival chance of about 15%. A remarkable man in many ways.

Edited to add

Not sure if he counts as "70s" because he worked from the early 60s through to the 2000s, but the late Mick Green of the Pirates also deserved far more recognition than he got.

Definite plus to many of the above especially Robbie Robertson and Todd Rundgren.
 
Also Dan Fogelberg, in my opinion. Listen to 1 & 2 back to back, Netherlands and Once Upon a Time...end up with False Faces. If you like them, throw in Promises Made or even the whole album :)

Frank Marino would definitely be on my list.


Not sure if he's underrated, but Tommy Bolin is one of my favorite rock guitar players from that era. (I do think that DP's Come Taste the Band is underrated).

tlw


On the plus side Wilko Johnson is at last getting the recognition he deserves, and in his case unlike many who only get recognised until after they die he was told he had terminal pancreatic cancer and organised an urgent farewell tour only to go through a roller-coaster couple of years ending in a huge and experimental operation and medical process with a survival chance of about 15%. A remarkable man in many ways.

 
+1.  Wilko's style of playing is deceptively difficult to emulate. The documentaries 'Oil City Confidential' and 'The Ecstasy of Wilko Johnson' are great viewing.