內戰後美國政府鼓勵民衆嚮西移民，還鼓勵修築鐵路，連接西部至經濟較發達的東岸地區。同時西部不少地方自1850年代都發現了金礦，吸引不少居於東岸的人淘金。1869年連接美國東西海岸的鐵路竣工，鐵路帶給美國原居民更多衝突，當當地土著首領和移民簽訂土地協議時，雙方對協議內容有不同理解。土著不能理解移民的私有財産理念，他們認為自己能在土地上打獵。一場為生存而戰的戰鬥開發了，許多土著買了槍械，襲擊移民宅地、馬車隊、鐵路和美國騎兵。而美國白人則大量屠殺土著賴以為生的野牛，由1860年的1500萬頭，到1885年衹剩下2000頭。土著被迫在貧瘠的土地上種植莊稼，他們不習慣種植，不能成為美國公民。土著與美軍長期戰鬥，使土著幾乎滅亡。1890年，最後一次戰鬥屠殺了200多名蘇族人。不久後所有的土著都被趕進保留地。土著美洲人的生活方式永遠結束。
這段時間美國利用其充足的人口、國內工業發展和一係列的國際投機活動，令美國國際影響力迅速攀升。在十九世紀末期，美國已成為世界工業大國，並出現新的科技（如電報和貝塞麥(轉爐)煉鋼法），興建更大的鐵路網絡。
在這個時期，美國也成為國際性參與者。在1900-1903徵服了超過一百萬的菲律賓人，並屠殺了大量菲律賓人。
Reconstruction took place for most of the decade following the Civil War. During this era, the "Reconstruction Amendments" were passed to expand civil rights for black Americans. Those amendments included the Thirteenth Amendment, which outlawed slavery, the Fourteenth Amendment that guaranteed citizenship for all people born or naturalized within U.S. territory, and the Fifteenth Amendment that granted the vote for all men regardless of race. While the Civil Rights Act of 1875 forbade discrimination in the service of public facilities, the Black Codes denied blacks certain privileges readily available to whites. In response to Reconstruction, the Ku Klux Klan (KKK) emerged around the late 1860s as a white-supremacist organization opposed to black civil rights. Increasing hate-motivated violence from groups like the Klan influenced both the Ku Klux Klan Act of 1870 that classified the KKK as a terrorist group and an 1883 Supreme Court decision nullifying the Civil Rights Act of 1875; however, in the Supreme Court case United States v. Cruikshank the Court interpreted the Fourteenth Amendment as regulating only states' decisions regarding civil rights. The case defeated any protection of blacks from terrorist attacks, as did the later case United States v. Harris. During the era, many regions of the southern U.S. were military-governed and often corrupt; Reconstruction ended after the disputed 1876 election between Republican candidate Rutherford B. Hayes and Democratic candidate Samuel J. Tilden. Hayes won the election, and the South soon re-entered the national political scene.
Following was the Gilded Age, a term that author Mark Twain used to describe the period of the late nineteenth century when there had been a dramatic expansion of American industry. Reform of the Age included the Civil Service Act, which mandated a competitive examination for applicants for government jobs. Other important legislation included the Interstate Commerce Act, which ended railroads' discrimination against small shippers, and the Sherman Antitrust Act, which outlawed monopolies in business. Twain believed that this age was corrupted by such elements as land speculators, scandalous politics, and unethical business practices. By century's end, American industrial production and per capita income exceeded those of all other world nations and ranked only behind Great Britain. In response to heavy debts and decreasing farm prices, farmers joined the Populist Party. Later, an unprecedented wave of immigration served both to provide the labor for American industry and create diverse communities in previously undeveloped areas. From 1880 to 1914, peak years of immigration, more than 22 million people migrated to the United States. Abusive industrial practices led to the often violent rise of the labor movement in the United States. Influential figures of the period included John D. Rockefeller and Andrew Carnegie.