Howells was born in London, the daughter of composer Herbert Howells, and was educated at St Paul's Girls' School, where her father worked as Director of Music. She made her first stage appearance at Dundee in 1939, in John Drinkwater's Bird in Hand, then moved to Oxford in 1942 and three years later made her London debut at the Embassy Theatre, Swiss Cottage. In 1947 she appeared in the comedy Jane at the Aldwych Theatre. After several years in the West End, and a brief stint on Broadway where she appeared in Springtime for Henry in 1951, she began to appear in films.
After the death of her father in 1983, Ursula Howells instigated the "Herbert Howells Society" and became a standard bearer for the promotion of his work. She financially supported the recording of his compositions and did much to encourage the publishing and promotion of church music.

Torture Garden
1967 · as Miss Maxine Chambers (segment 3 "Mr. Steinway")

Dr. Terror's House of Horrors
1965 · as Mrs. Deirdre Biddulph (segment 1 "Werewolf")

The Long Arm
1956 · as Mrs. Elliot / Mrs. Gilson

Crossplot
1969 · as Maggi Thwaites

Time After Time
1986 · as May Swift

Number Three
1953 · as Maureen Dartington

Flesh and Blood
1951 · as Harriet Marshall

Assignment K
1968 · as Estelle

Mumsy, Nanny, Sonny & Girly
1970 · as Mumsy

Account Rendered
1957 · as Lucille Ainsworth

The Cold Room
1984 · as Headmistress

The Enigma
1980 · as Mrs Fielding

The Constant Husband
1955 · as Ann

Father Dear Father
1973 · as Barbara

When the Actors Come
1978 · as Barbara Karolina

The Tichborne Claimant
1999 · as Lady Doughty

I Believe in You
1952 · as Hon. Ursula

A Pinch of Snuff
1994 · as Alice Andover