With the Union safely reunited, albeit tenuously, Grant set his sights on the Presidency. Everyone around him knew it was the only logical job for the man viewed as the greatest military hero since 1781. However, the country needed to wait for his ascendance. Unlike his Lieutenant Generalship, his next step could not be created by a special act of Congress. Grant settled in Washington, and everywhere he traveled dinners celebrated him and crowds adored him. Through the fall of 1865, he toured the North and then set off on a "home-coming" tour of Ohio. In November 1865, at the request of President Andrew Johnson, Grant made a cursory tour of the South–after which he advocated the continuation of the Freedman's bureau and of the military posts in the South.

As 1866 began, and Congress and Johnson argued over Reconstruction policies, Johnson was left with the conundrum of Grant's presence. The general was far too popular to get rid of, yet the longer he was around Washington the bigger a threat he became to Johnson's political future. Grant, who had always favored a generous peace, tried to work out a compromise position between the radical Republicans and Johnson–especially after Grant's tour of the South found the region's people yearning for a peaceful settlement. However, Johnson's pro-Southern policies, the outbreak of renewed violence towards blacks, and rioting in the former Confederacy disturbed Grant, who had been promoted to a full generalship by Congress in 1866. A tour of the country by Johnson, accompanied by Grant, turned into a debacle, as the country reacted unfavorably toward Johnson. Grant's advisers feared that such feelings would rub off on Grant if he and Johnson were seen together much more.

On August 12, 1867, Johnson suspended Secretary of War Edwin M. Stanton and asked Grant to act as interim Secretary. The next five months were among the most uncomfortable of Grant's political career. When the Senate refused to accept the suspension of Stanton, Grant resigned. He later supported the impeachment attempts against Johnson, and in the summer of 1868 submitted his name for the Presidential nomination of the Republican Party. Grant watched nervously from Galena, Illinois as the election results came in. He easily defeated the Democratic candidate, Horatio Seymour, garnering 214 of the 294 electoral votes.

On March 4, 1869, Grant became the eighteenth President of the United States. He privately selected a Cabinet, relying on few others to help him. Unfortunately, Grant did not select strong leaders, but rather close personal friends, like Secretary of War John A. Rawlins, and campaign contributors like the wealthy Secretary of the Navy, Adolph Edward Borie. While Grant's selections seemed like a good idea at the time, the appointments created an environment in which few members of the administration fully understood what happened in their departments.

Overall, the greatest general the nation had ever seen soon became one of its worst Presidents. Grant's motto, "Let us have peace," was supported by no policy initiatives to give it teeth. Furthermore, Grant expressed little desire to make policy or enforce it over the will of the American people. Thus, when considering his Presidency, it is important to make the distinction between what Grant himself did and what his administration did–as Grant rarely had much knowledge of what the people under him were doing.

Luckily, Grant's lack of knowledge about the power of the executive branch was precisely what Americans wanted in 1868. They had grown tired of the politicking and tricks of Andrew Johnson and wanted a President who was subordinate to the legislature–exactly Grant's approach. He saw the Presidency as an administrative position in which he would do no more or less than Congress asked of him. Perhaps most dangerous was the fact that Grant saw the Presidency as only the latest in a long series of gifts from the American people for his victories; he never realized that these gifts carried a price. As a poor man for most of his life, Grant loved the high life that the White House offered, in which rich and powerful men came to ask him for favors and bestowed money and gifts upon him. Likewise for Julia, the White House was a dream come true.

America stood at an important crossroads during Grant's Presidency. Settling a dispute with Britain over the Northwest Territory, America for the first time understood its borders exactly and undisputedly. It had a stable currency for the first time, and the Gilded Age was on the horizon. America needed a forward thinker who could rebuild–and Grant was not the right man.

The first major problem of Grant's administration arose over gold. Grant obeyed the conservative Republicans who believed that the United States needed to rid itself of the paper money issued during the war. Grant therefore signed legislation declaring the government's intention to cash in the paper money for gold–and he ultimately got duped by two speculators, Jay Gould and Jim Fisk. The Secretary of the Treasury controlled about a hundred million dollars of gold, and each month he was to sell some of it retire the paper money. Gould and Fisk realized that if they knew what the Secretary would do–or, even better, prevent the Secretary from doing anything at all–they could corner the gold market and make a killing. They invited Grant to New York and wined and dined him, gradually convincing him that the fall crops would fetch better prices if the government did not release any gold–but as they spoke to him they madly bought gold. Grant concurred and the price of gold began to rise, higher and higher.

Eventually Grant figured out the plot. He fired the low-level officials Fisk and Gould had bribed, ordered the prompt sale of large amounts of treasury gold. On Black Friday, September 24, 1869, the corner was broken, and the price of gold fell from $162 an ounce to $135. The damage had been done, however, as Grant's political capital never really recovered from his first stumble. After all, the people he had hurt as an unknowing accomplice in the swindle were the people who were just like him: struggling businessmen and farmers who could never have afforded gold themselves.

Next, Grant made his single independent foray into foreign policy. He tried to annex Santa Domingo, now the Dominican Republic. The president of the small country wanted to make some money and the U.S. Navy wanted a coaling base in the area. Grant also believed he could move the freed slaves there and thus create three or four all-black free states in which they could start over. He believed such an act would do the former slaves justice. However, Grant's attempts showed a complete lack of understanding of the political process. He sent only an aide to the island to work out the transfer, bypassing Congress and his own Secretary of State. When Grant brought the treaty to Congress, Senate Charles Sumner, the chair of the foreign relations committee, blocked it and denounced Grant's attempts.

Grant's administration slowly came to represent the loss of political integrity that now pervaded the system–leading to the rise of "bosses" and "machines" such as Tammany Hall in New York. Although Grant himself remained clean, and could not be faulted for anything more than ignorance, his record became permanently tarnished. Grant even tried to have the spoils system ended, but he was no match for seasoned Congressional politicians.