Admitted to the bar in 1832, Strong practiced law in Reading, Pa., and served in the U.S. House of Representatives (1847–51). While sitting on the Pennsylvania Supreme Court (1857–68), Strong, a Democrat but a firm supporter of the Union, changed his political affiliation and became a Republican.

On Feb. 7, 1870, President Ulysses S. Grant (Grant, Ulysses S.), also a Republican, nominated Strong to succeed the retiring justice Robert C. Grier, a Democrat. At the same time, Grant named Joseph P. Bradley (Bradley, Joseph P.) to fill a new seat on the court, which had been made available by a Congressional authorization enlarging the number of the justices from eight to nine. The circumstances of new appointments were such that Grant was charged with a court-packing scheme, and internal dissension with the court was exacerbated. The very day that the two appointees were nominated, the Supreme Court announced its decision in Hepburn v. Griswold (1870), a case that involved the constitutionality of the Legal Tender Act of 1862. The court, in a 5–3 vote (including a vote for the majority by the ailing Grier), struck down the Legal Tender Act, thus denying Congress the power to issue paper currency as legal tender.

The following year, in Knox v. Lee and Parker v. Davis (1871), the newly formed court overturned the Hepburn decision by a vote of 5–4. Strong spoke for the majority, upholding the government's power to enact legal-tender legislation and defending such power under the “necessary and proper” clause of the Constitution. The abrupt reversal of a major decision so soon after the enlargement of the bench renewed the charges against Grant. Despite this controversy, which overshadowed Strong's appointment to the high court and his first major decision, he served with distinction for 10 years, winning the respect of the legal community for his ability and integrity.