She began her missionary work as a teacher in 1928 in Calcutta, India.  She served her mission by walked into the slums and gathering together unschooled children and teaching them literacy and hygiene. In 1948 the Vatican granted her permission to leave her convent in order personally to tend the homeless, starving, and sick in Calcutta's slums.
Revelation. Once Mother Teresa saw how one boy had brought a wheelbarrow with his dying mother and left her near the hospital gates. The woman's body had been rotted and she wasn’t taken to the hospital. Teresa prayed, washed the woman and took care of her. She died in her arms with a smile on her face.
Then on Sept. 10, 1946 during her walk she had a revelation and realized that she had to leave the Order of Loreto and to settle in the slums of Calcutta in order to help the misfortunate and poor people.
On August 16, 1948 Mother Teresa, aged 38, having changed in a cheap sari bought on the market, left her abode owning only five rupees. Members took an fourth vow in addition to the traditional ones of poverty, chastity and obedience, which was to give free service to the most abjectly poor.  She founded the Missionaries of Charity, which now operates schools, hospitals, orphanages, and food centers worldwide.
Others joined her and in 1950 her new Order, the Missionaries of Charity  received official status as a religious community within the Archdiocese of Calcutta.
Later another two orders have been founded: Missionary Brothers of Charity (1963) and the International co-Workers of Mother Teresa (1969). The missions were opened in Venezuela (1965), Ceylon (1967), in Rome and Tanzania (1968), and  in Cuba (1986).  Accompanied by Red Cross workers, she traveled through the war zone to the devastated areas. 
In 1991, Mother Teresa returned for the first time to her homeland and opened a Missionaries of Charity Brothers home in Tirana, Albania.

Although Agnes was born on August 26, she considered August 27, the day she was baptized, to be her "true birthday." Her first name - Gonxhe meaning "rosebud" in Albanian.
Her father died in 1919 when she was eight years old.  After that her mother raised her as a Roman Catholic. When she was 12 years old Agnes was fascinated by stories of the lives of missionaries and was convinced that she should commit herself to serving people.  She left home at age 18 and never again saw her mother, sister and brother. 
In 1928 Agnes Gonxhe Bojaxhiu went to the Loreto Abbey in Rathfarnham, Ireland and joined the Sisters of Our Lady of Lareto, a Catholic order that did charity work in India. At that time she chose the name Teresa after Thérèse de Lisieux, the patron saint of missionaries and  took her first vows.
She arrived in India in 1929, and began her novitiate in Darjeeling, near the Himalayan mountains. She took her solemn vows on May 24, 1931 and on May 14, 1937, while serving as a teacher at the Loreto convent  girls' school in eastern Calcutta.
Although in her later years she suffered from a worsening heart condition, Mother Teresa continued to serve the poor and sick and also lead an active social life, spoke out against divorce, contraception, and abortion.
She died on 5 September 1997 at the age of 87 years old.  
Remains: Buried, Mother House Convent, Calcutta, India.