Many influences carved Carnegie's personality, including his Scottish roots, his family's struggle with poverty, the egalitarian spirit of his extended family, and even the city of Pittsburgh, one of centers of 19th century industrial America. Many people, from industrialists to philosophers, also shaped Carnegie the man. Yet the most influential of all was most likely his mother Margaret, the backbone of his family, who resided with Carnegie until her death when Carnegie was still a 51-year old bachelor.

Carnegie was the first to acknowledge the role his mother played. "Perhaps some day I may be able to tell the world something of this heroine, but I doubt it," Carnegie wrote in his autobiography. "I feel her to be sacred to myself and not for others to know. None could ever really know her--I alone did that. After my father's early death she was all my own."

Andrew, her first son, was born in Scotland in 1835 to the 25-year old Margaret. By the mid-1840's, the family was sliding into abject poverty. William, Margaret's husband, was a hand weaver who was losing his trade to the new power-driven factory looms. The family had to leave their large house and move back to small quarters. Margaret opened a small food store to add to the family's income. As of the winter of 1847-1848, it was not at all clear that the family would survive Scotland's industrialization.

Carnegie's mother taught the young Carnegie the frugality that he would become famous for later. One day in school he quoted a proverb that his mother had repeated often: "Look after the pennies and the pounds will look after themselves." His classmates laughed at him, unaware that the principle would help make Carnegie one of the richest men in the world.

Many of Margaret's countrymen fled to the promise of America, including two of Margaret's sisters and a brother who had emigrated to Pennsylvania and Ohio. One of her sisters wrote home to Margaret to tell her of the better life that awaited her across the ocean:

This country is far better for the workingman than the old one, and there is room enough to spare, notwithstanding the thousands that flock into her borders every year. As for myself, I like it much better than at home, for in fact you seem to breathe a freer atmosphere here; but as my husband says, no wonder women like it, for they are so much thought of in this country.

Margaret followed her two sisters to Pittsburgh. Her husband took up the grueling factory work at a nearby cotton mill, but he soon left it to return to his handloom to make tablecloths that he sold door to door. Mrs. Carnegie, once again picking up the financial slack, took up sewing shoes for a shoemaker in the neighborhood. During the time his family was still poor, Andrew found his mother crying about the family's struggles. 

"Some day," Andrew promised, "I'll be rich, and we'll ride in a fine coach driven by four horses." Margaret was quick to reply: "That will do no good over here, if no one in Dunfermline can see us." It was then that Carnegie resolved that he would one day revisit Dunfermline with his mother in a coach that the entire town would notice.

Even before Margaret's husband died in 1855, the family tied its financial star to Andrew, who supported his mother and his younger brother Tom. In 1859, Andrew was appointed superintendent of the Western Division of the Pennsylvania Railroad Company. His salary was raised to $125 a month, which put his family into a comfortable position. Andrew moved with his mother and 16-year old brother, Tom to the East Liberty neighborhood of downtown Pittsburgh, a fashionable area.

In 1867, Andrew and his mother moved to New York City, his brother Tom having started a family of his own. After a few years at the St. Nicholas Hotel, the two moved uptown to the Windsor Hotel on Fifth Avenue, then a more residential neighborhood that suited his mother. Often away for trips, Carnegie knew his mother could be cared for by hotel staff, a luxury that the matriarch in the family enjoyed fully. Andrew sometimes expressed a desire to have his own dining room to entertain guests, but Mrs. Carnegie liked where she lived, once responding, "We cannot do better than this."

Andrew's mother not only rode his coattails; she also was possessive of her son. In 1880, when Margaret was approaching 70 and her son was already 45, that a serious threat was made to her dominion. Andrew became friendly with an intelligent woman named Louise Whitfield, a daughter to one of his business associates and 21 years Carnegie's junior. The two got to know each other while horseback riding through Central Park and over time they became more than friends.

In 1881, Carnegie now a very rich man, decided to follow through on his promise to take his mother back to Dunfermline in style, and he wanted to take Louise as well. Andrew asked his mother to convince Louise's family that the trip would be appropriate for a young, single lady. Mrs. Carnegie went to visit Louise and her mother, but her intention appears to have been to subvert Louise's voyage. When Mrs. Whitfield asked the elderly Mrs. Carnegie whether such a trip was appropriate for a young unattached woman, Andrew's mother answered with conviction: "If she were a daughter of mine she wouldna' go." Louise, dejected, stayed behind.

Mother and son, accompanied by friends, returned now to the home the Carnegie family had abandoned 33 years ago. This time they went back as rich benefactors, as Carnegie had bequeathed a new library to his first home. Thousands of townspeople greeted them with frenzied cheers, hanging banners such as "Welcome Carnegie, Generous Son." The Carnegies had left Dunfermline in steerage, but they came back to town in a magnificent coach, as Carnegie had promised his mother. As Margaret rode through the town in her carriage, she cried tears of joy. It may very well have been Margaret's crowning hour, a final triumph for the woman who as a young mother left her home fearing destitution.

Despite the snub, the romance between Louise and Andrew continued. In September, 1883, the two were secretly engaged. Yet when it came to setting a date for the wedding, Carnegie balked. The two agreed to call off the engagement. 

Only after Margaret's death in November, 1886, could Carnegie commit to marrying Louise. Carnegie, recuperating from typhoid fever, wrote her soon after he lost his mother: "It was six weeks since the last word was written and that was to you as I was passing into the darkness. Today as I see the great light once more my first word is to you. . . Louise, I am wholly yours--all gone but you. . . I live in you now. Write me. I only read yours of six weeks ago today. Till death, Louise, yours alone."

The two married and had a daughter, who they named Margaret. After Andrew died, Louise confided to one of Carnegie's biographers that her mother-in-law was the most unpleasant person she had ever known, an assessment the adoring son Andrew would certainly have disputed.