She reportedly insisted that his speech announcing his plans include the definitive phrase "I shall not seek and I will not accept" his party's nomination.
Mrs Jonhnson was thrust into the position of first lady after the assassination of John F Kennedy in 1963.
"[She was] a steady, gentle presence for a mourning nation"
The Johnsons were two cars behind Kennedy in the motorcade as it travelled through Dallas.
Paying tribute to her on Wednesday, George Bush, the US president, said Americans "will always remember her with affection" pointing to what he said was her strength in the days after the assassination.
She "was a steady, gentle presence for a mourning nation in the days that followed," he said in a statement.
Ladybird Johnson outlived her husband by nearly 35 years - he died in 1973.
The former first lady had a stroke in 2002, which left her partially paralyzed, and was again in the hospital recently with a fever.