Here are a couple of random facts and websites with more information:

Marriage record for John Neff and Mary Barr
"22 February 1822. On Tuesday the 12th inst. (married) by the Rev. Mr. Boyer, Mr. John Neff, Jr. of Strasburg twp. to Miss Mary Barr, daughter of Christian Barr, dec'd., late of Bart twp."
(Source: The Lancaster Journal, Lancaster, Pa.; Feb. 22, 1822.)

History of Utah: Biographical

By Orson Ferguson Whitney

Page 105

JOHN NEFF

The founder of Mill Creek, where he built the first grist-mill south of Salt Lake City, the subject of this narrative was a native of Strasburg, Lancaster county, Pennsylvania, and was born September 19, 1794. His parents were John and Barbara Herr Neff. They were wealthy and gave their son a good education, both in English and in German. He was particularly noted for his fine penmanship. His boyhood and early manhood were passed in his native place, where he became a farmer and stock raiser and withal a shrewd business man. His father and Mr. Frank Kendig were the owners of a woolen factory near hie home, but failed in business when John was quite young. He bought them out, thus obtaining his start in life, securing the factory at a very low figure, as the neighbors, out of respect, would not bid against the son in purchasing his father's property. While carrying on farming and stock-raising he also had a distillery and manufactured liquor.

In politics he was a Whig, and was intimately acquainted with President Buchanan, Thaddeus Stevens and other leading politicians of the time. In his youth he saw General LaFayette, during his last visit to America, and would often describe the personal appearance of the distinguished Frenchman. He as well as his parents was highly respected, their names being among the most honored in their neighborhood.

The first Mormon meeting attended by John Neff resulted in his conversion to the faith of the Latter-day Saints. The meeting was held in a schoolhouse near his home in Lancaster county, and the preacher was Elder Henry Deem. Soon after this he was baptized, and from that hour until his death his religion was to him the most precious thing in existence.

In the year 1821 he married Mary Barr, daughter of Christian and Susanna Breneman Barr, who was ever a faithful and devoted companion, united with him in all things. In the spring of 1844, accompanied by his wife and child, a daughter named Barbara, he visited Nauvoo, Illinois, for the purpose of seeing the Prophet Joseph Smith, and purchasing property in the City of the Saints, with a view to moving there at an early day. The visitors were warmly welcomed by the Prophet, whom they heard many times in public and in private. On one occasion he said in their hearing that he should not live long. They stayed during their visit at the Mansion House, and left Nauvoo about six weeks before the martyrdom.

Returning to their home in Pennsylvania, they remained until the summer of 1846, when, having disposed of their property at a great sacrifice, they set out to join their people, who were then in the midst of the exodus from Illinois. Mr. Neff had an excellent outfit, for he was still well-to-do, notwithstanding his financial sacrifices. His teams, carriages and equipment were of the best.

He passed the winter of 1846-7 on the Missouri River, where he outfitted Orrin Porter Rockwell, one of the Pioneers; and after their departure for the West he made preparations to follow them in the first company of emigrants to Salt Lake valley. He was organized in the division commanded by Jedediah M. Grant, and between him and Captain Grant, Uncle John Young and other prominent men there sprang up a warm friendship. He entered the valley on the 2nd of October.

He first made his home in the "Old Fort," but early in the spring of 1848 he began the construction of a grist mill below the mouth of Mill Creek canyon. During the summer he moved out to that vicinity, where sprang up a settlement of which he was virtually the founder. His mill was completed and began to grind during the winter of 1848-9.

Father Neff was active from the first in developing the agricultural resources of the country, planting potatoes from seed brought by the Pioneers. During the dark days that followed, when the crops of the settlers were threatened and at times devoured by crickets and grasshoppers, he was always hopeful and predicted the prosperity that would follow. He was a generous and charitable man, freely imparting of his substance for the relief of the poor and needy.

Modest and retiring, he shrank from public life and notoriety, and the offices held by him were few. In the Church he was a High Priest, and he accompanied President Young and party on a mission to Salmon River. He also acted as a commissioner to locate University lands. This seems to have been the extent of his official service, though not by any means the limit of his usefulness to the public.

John Neff was the father of five sons and five daughters. His youngest son and name-sake is now Bishop of East Mill Creek. The honored sire departed this life May 9, 1869, at his old home in the settlement that he founded.

There is a very good history of John Neff at http://www.eternal-links.net/Family_History/ngf/11/2/history/John%20Neff%20II.html.

Here is an article from the Neff Times about the Flour Mill John Neff built.
http://www.eternal-links.net/Family_History/ngf/11/2/mill/Mills.html

This site is a picture of Mary Barr http://content.lib.utah.edu/cdm4/item_viewer.php?CISOROOT=/USHS_Class&CISOPTR=2747&CISOBOX=1&REC=1

Enjoy learning and let me know if you find more information. Alice Neff, alice@neff.ws