Teasdale had poor health for most of her life, and it was only at age 9 that she was well enough to begin school. In 1898 she began attending Mary Institute, but switched rapidly to Hosmer Hall in 1899, where she finished in 1903.
Teasdale's first poem was appeared in Reedy's Mirror, a local newspaper, in 1907. Her first collection of poems, Sonnets to Duse and Other Poems, was published that same year. Teasdale's second collection of poems, Helen of Troy and Other Poems, was published in 1911. It was well received by critics, who praised its lyrical mastery and romantic subject matter.
In 1913 Teasdale fell in love with the poet Vachel Lindsay. He wrote her daily love letters, but nevertheless she married Ernst Filsinger, a rich exporter, in 1914. Teasdale and Lindsay remained friends throughout their lives.
Teasdale's third poetry collection, Rivers to the Sea, was published in 1915. The collection received further critical praise.
In 1918, her poetry collection Love Songs won three awards: the Columbia University Poetry Society prize, the 1918 Pulitzer Prize for poetry and the annual prize of the Poetry Society of America. She was not happy in her marriage, divorcing Filsinger in 1929. In 1933, she committed suicide by overdosing on sleeping pills. Her friend Vachel Lindsay had committed suicide two years earlier.
In 1994, she was inducted into the St. Louis Walk of Fame.
She is interred in the Bellefontaine Cemetery in St. Louis.
A common urban legend surrounds Teasdale's suicide. The legend claims that her poem "I Shall Not Care" (which features themes of abandonment, bitterness, and contemplation of death) was penned as a suicide note to a former lover. However, the poem was actually first published in her 1915 collection Rivers to the Sea, a full 18 years before her suicide.:
I shall not care to have him then,
I shall be bitter and a-cold —
It grows too late for frolicking
When all the world is old.
Then little hiding Love, come forth,
Come forth before the autumn goes,
And let us seek thro' ruined paths
The garden's last red rose.
I shall have peace, as leafy trees are peaceful
When rain bends down the bough,
And I shall be more silent and cold-hearted
Than you are now.
But they float away — for who can hold
Youth, or perfume or the moon's gold?
And Spring herself, when she woke at dawn,
Would scarcely know that we were gone.
With earth hidden and heaven hidden,
And only my own spirit's pride
To keep me from the peace of those
Who are not lonely, having died.