Timeline:
1889: John Sr. born
1911: John Jr. born
1912: Herbert born
1913: Harry born
1915: Donald born
1922: group of 4 brothers form singing group
1925: first professional work
1931: radio contract, etc
1932: first movie appearance, in "The Big Broadcast"
1934-1935: touring Europe and England, John Jr. gets ill
1936: John Jr. dies of Tuberculosis. His bass singing is replaced by father John Sr, his guitar playing is replaced by Bernard Addison.
1942: "Paper Doll" recorded, becomes biggest Mills hit, selling six million
1943 or 1944: Harry drafted
1952: "The Glow Worm": last #1 hit
1956: John Sr. retires from group, which continues as trio
1967: John Sr. dies
1968: last top-40 hit: Cab Driver
1981: last concert, in Atlantic City
1982: Harry dies
~1986: Donald starts touring with son John
1989: Herbert dies
1999: last survivor, Donald, dies
2000-present: Donald's son John Mills, and former Platter Elmer Hopper perform as "Mills Brothers"

Rough interpretation of their periods:

1920's: Learning singing from parents, barbershop singing.
1930's: Hot jazz style, imitated musical instruments, scat singing. In second half of decade are in Europe (esp. Britain and Scandinavia) about as much as USA. Record usually alone but some with Louis Armstrong, Ella Fitzgerald, Al Jolson.
1940's: They adopt a smoother style, conforming with crooners (e.g. Bing Crosby) of the time. Many songs begin with a couple soft guitar cords, then Donald with a very smooth solo, then tempo shifts up and Harry has a solo in a more swinging style. Seldom do the instrument imitations, guitar is soft accompaniment to harmonious singing.
1950's: Record more with big bands, incl Sy Oliver, Sonny Burke, Tommy Dorsey and others. Orchestras play a big role, guitar less, and there are many bars where the Mills are tacit.
1960's: Continue developing integration of their singing with orchestras. Record with Count Basie.
1970's: No longer having hits, more of a "nostalgic" approach, i.e. singing their old hits, usually with orchestra.


Thoughts on what makes the Mills so good:

Some people now emphasize racial injustice. Rap music does more than its share, and it is often hateful, angry music. The Mills' never is. Regarding race, Donald Mills said "As for any problems, we never had any problems because we never played anywhere our own people weren't welcome." Brian Epstein said when he first met the Beatles (he became their manager), he was most impressed by their personal charm. There are great musicians who are not personally charming, but I do think a musician with charming manner usually has it impact their music. The easy-going nature of the Mills, especially Donald, comes across in their music.

Perhaps more than any well known musical group, they developed as musicians together. Tight natural co-ordination and easy co-operation.

For most of their career, they are more vocally oriented than almost anyone: no instruments except a low-key guitar, and they can carry it well.