Wallace was diagnosed with amyotrophic lateral sclerosis (ALS) in 1964. He consulted numerous specialists and tried various methods of treating the disease, stating, "I look on myself as an ALS guinea-pig, willing to try almost anything". He died in Danbury, Connecticut, on November 18, 1965, at the age of 77. His remains were cremated and the ashes interred in Glendale Cemetery in Des Moines, Iowa. Due to his successful business career and wise investments, he left an estate valued at tens of millions of dollars.