John M. Dennison was born between 1856-1857 in Cooks Run, Clinton County, Pennsylvania to Hiram and Louisa (née Whaling) Dennison. His biological mother passed away between 1856-1859. John's father (Hiram) remarried to Elizabeth Weaver on February 17, 1859.

1860 Census: Chapman, Clinton, PA
Name: John Dennison
Age: 3
Birth Date: abt 1857
Birth Place: Pennsylvania
Post Office: Youngwomanstown

Household Members
Hiram Dennison 51 (Father)
Elizabeth Dennison 47 (Step Mother)
Jane Dennison 15 (Sister)
Andrew Dennison 9 (Brother)
William Dennison 8 (Brother)
Mary Dennison 5 (Ella or another sister?)
*John Dennison 3

1870: Keating, Clinton, PA
Name: John Denison [Dennison]
Age in 1870: 13
Birth Date: abt 1857
Birthplace: Pennsylvania
Post Office: Westport
Occupation: At Home
Attended School: Yes

Household Members
Hiram Denison 65 (Father)
Elizabeth Denison 57 (Step Mother)
Andrew Denison 21 (Brother)
William Denison 18 (Brother)
Ellen [Ella] Denison 15 (Sister)
*John Denison 13

John married Alice Paulina (née Nelson), daughter of David and Belinda Nelson at his father's residence on April 30, 1876.

The Clinton Democrat, Thur., May 11, 1876.
Married.
On the 30th ult., at the residence of Mr. Hiram Dennison, of Noyes twp., by the Rev. McCormick Graham, Mr. John Dennison and Miss Alice Nelson, both of Cook's Run, this county.

1880 Census: Noyes, Clinton, PA
Name: John M. Dennison
Age: 24
Birth Date: Abt 1856
Birthplace: Pennsylvania
Marital Status: Married
Spouse's Name: Alice D. [P.] Dennison
Father's Birthplace: Scotland
Mother's Birthplace: New York
Occupation: Farmer

Household Members
*John M. Dennison 24 Self (Head)
Alice D. [P.] Dennison 21 Wife
Flora B. Dennison 5 Daughter
Dora Dennison 2 Daughter
Elizabeth Dennison 58 Step Mother

In November 1882, John was involved in a hunting accident, which took the life of George Brinton McClellan Werts (Find a Grave Memorial 147733230). George Brinton was the brother of John's in-laws, John S. Werts (Husband of Ella Dennison) and Sarah Frances Wertz (Wife of Andrew Oliver Dennison). This MIGHT explain why he relocated from the Cooks Run area of Clinton County, PA to Shamokin, Northumberland County, PA between 1884-1885.

1889-1891: John was residing on S. Pearl Street in Shamokin, PA.
(Boyd's Directory of Shamokin, PA)

September 28, 1897
John remarried to Ethel "Effie" (née) Arnold while still legally married to his wife, Alice.

The Daily Item, Sunbury, PA
Wednesday, September 29, 1897, Page 4
John M. Denison and Miss Effie Arnold, of Shamokin, were married by Justice Kelley Tuesday afternoon.

October 6, 1897
John was arrested on Wednesday, October 6, 1897.

Mount Carmel Item, Mount Carmel, PA
Saturday, October 9, 1897, Page 1
John M. Dennison, the much married Shamokinite, was lodged in jail Wednesday evening on four charges-bigamy, seduction, adultery and perjury. Mrs. Dennison No. 2 was held in $300 bail for court on a charge of adultery.

The Daily Item, Sunbury, PA
Tuesday, December 7, 1897, Page 1
John M. Dennison, the Shamokin scallawag
who came to Sunbury and procuring a license was married by Justice Kelley to a Shamokin girl while he had a wife in that place, plead guilty to four serious charges, two of which were bigomy and perjury.

November 11, 1898
John was released from jail on the morning of Friday, November 11, 1898.

Northumberland County Democrat, Sunbury, PA, Thursday, November 17, 1898
JAIL NOTES.
John Dennison of Shamokin, who has been in jail eleven months for bigamy, was discharged Friday morning.

1899
A deed from Alice Dennison, Hiram Hollingsworth and Jane Hollingsworth to Harry W. Wilson indicated that John deserted his family and his whereabouts were unknown. This deed is located in Clinton County (PA) Deed Book 61, Page 340. Dated March 30, 1899 and recorded on June 28, 1899.

1900
In the 1900 US Census, John's wife (Alice) was listed as "widowed" and living in Shamokin, PA with their children.

NOTE: John's date/place of death and burial remains unknown at this time.