Answer by taking a quote from the following article:

Winfield Scott (June 13, 1786 - May 29, 1866) was a United States Army general and the unsuccessful presidential candidate of the Whig Party in 1852. Known as "Old Fuss and Feathers" and the "Grand Old Man of the Army", he served on active duty as a general longer than any other person in American history, is rated as one of the Army's most senior commissioned officers, and is ranked by many historians as the best American commander of his time. Over the course of his 53-year career, he commanded forces in the War of 1812, the Black Hawk War, the Mexican-American War, and the Second Seminole War. He was the army's senior officer at the start of the American Civil War, and conceived the Union strategy known as the Anaconda Plan, which was used to defeat the Confederacy.

During the Mexican-American War, Major General Scott was appointed by President James K. Polk to lead an army of regulars and volunteers to the Rio Grande for a hasty campaign. During the planning and initial movement, worsening political tensions between Scott and the president led to a public shellacking and relief of Scott as field commander. With reluctance, Zachary Taylor was charged with leading the charge to the Rio Grande.  While Taylor was largely successful in securing the northeastern provinces of Mexico after war broke out, it became obvious by mid-1846 that the Mexicans would not surrender the captured territories without a direct assault on their capital. Deeming an overland campaign from northeastern Mexico infeasible (requiring marching over 560 mi (901 km) of Mexican desert), Scott planned an expedition to the Gulf port city of Veracruz. As Taylor gained notoriety for victories in northeastern Mexico, Polk became increasingly reluctant to position him for a presidential run post-bellum. Further, Polk and his cabinet had reasonable doubts whether Taylor could lead the complex operation. Left to choose between Taylor and Scott, Polk reluctantly chose Scott at the behest of his cabinet.  Even while Scott was en route to the theater of operations, Polk continued to search for a fellow Democrat to command the expedition in lieu of Scott. Senator William O. Butler and Robert Patterson were both selected as early options, but neither were deemed acceptable by Congress. Patterson, who was Irish-born and not eligible to be President, was dismissed early on as a suitable choice. Butler's capacity to command an army was questionable at best, as he had never seen combat and lacked experience in the regular Army.

Was he appointed by anyone is?
Left to choose between Taylor and Scott, Polk reluctantly chose Scott at the behest of his cabinet.