Major with a flat 7 — one note swapped, and the whole scale takes its boots off. The leading tone was the engine of arrival; lower it a whole step below home and nothing insists anymore, which is why Mixolydian vamps roll instead of resolve. Summon it with 1⁷ → b7 → 4: the b7 major chord is the flag, impossible in plain major and inevitable here, and the tonic's own dominant seventh doubles down on the same note. This is the sound of classic rock choruses, fiddle tunes, and every one-chord funk groove that never wants a downbeat amen.
The b7 — a whole step under home instead of a half; the leading tone quits and the music relaxes its shoulders.
Your tonic is the 5 of the parent — the major key a perfect 4th up shares every note. G Mixolydian is C major's set with G as home.
One chord kills it: a major 5 (or 5⁷) drags the natural 7 back in and the vamp becomes a major key mid-phrase. If the song needs a real 5 → 1 cadence, treat Mixolydian as a section color, not the law.
'Old Joe Clark' (it's in the songbook) wears it as a fiddle tune; the Beatles' 'Norwegian Wood' verses live here; 'Sweet Home Alabama' is the famous standing argument between D Mixolydian and G major.